Bump protobuf from 3.15.5 to 3.15.6 in /requirements

Bumps [protobuf](https://github.com/protocolbuffers/protobuf) from 3.15.5 to 3.15.6.
- [Release notes](https://github.com/protocolbuffers/protobuf/releases)
- [Changelog](https://github.com/protocolbuffers/protobuf/blob/master/generate_changelog.py)
- [Commits](protocolbuffers/protobuf@v3.15.5...v3.15.6)

Signed-off-by: dependabot[bot] <support@github.com>

requirements/prod.txt

@@ -500,27 +500,27 @@ google-resumable-media==1.2.0 \
 proto-plus==1.14.2 \
     --hash=sha256:f055028886bbe1de17d90c041cca4174fb0dcff5a1641a522c7e89b19cdbe97d \
     --hash=sha256:cd31eba8161b7e11fecc4b4ba306100239081a78796dd56c93940547c2a23d7d
-protobuf==3.15.5 \
-    --hash=sha256:d26ed8dbdbe6b62cd24173c9ceb7588ae7831eec172ac002b095af091db01196 \
-    --hash=sha256:9133b39924485ae43c02fc8274e57e5aa1706ad0970de49c72cfb8c0854d5f89 \
-    --hash=sha256:6bb44c15c98091e926a98362bff7fb24338bdf4001a6614834b8414c3b8593ee \
-    --hash=sha256:2d4cede5f2f2514df4a1eda1424a14d46daa5ea57963a1ea0fdab8d74ca2f9cd \
-    --hash=sha256:ab735b3a4342004afa60ff580ce2be0f2aa784f1f69ee7f08a23ef26d22d811d \
-    --hash=sha256:a390e4bbb8232945fc8e4493c8b70949423a6dacee6f0353021b59c40b039e25 \
-    --hash=sha256:dc7191b2e3361fdf2979e78a120a3a40e9d811318f6b2629036f53d9cb041c09 \
-    --hash=sha256:762f6b9fb8025db34f762a860fd2b1473dfc84bcd0c3e4f396a695c83d733729 \
-    --hash=sha256:d1aab4d0aed36f7873734a243b46786d407cfa1010fae886249db56a1493a057 \
-    --hash=sha256:119b4d308c87e833b6265b3922d5f5927e9d804605fcb1c1f771aa4d17e03591 \
-    --hash=sha256:c5b37b117ef89431149883d9b867c341a01f835142864722534885dcc1db6b1b \
-    --hash=sha256:f75aa0483fec2e4208bd4be18da0e3d7161dc74c65b6d6108f5968a8fe53a8ce \
-    --hash=sha256:5d52d89e26adf0ba65193b6be39025c7766740ccc57fe9d10ddb709220b360d9 \
-    --hash=sha256:87b5bc2ff944810a918628fc1f45f766acab23e1fecb0634fcf86cda554b30c4 \
-    --hash=sha256:282385b8dd168b0f71f2ffca74c1fb39377f42217830ab492a0b64cbe14f86c1 \
-    --hash=sha256:9f4445f197f779cd5b37c9d5d4aeb0d1999c1df7d143a9bce21d03dac8dba205 \
-    --hash=sha256:ac7c7a2b271307787ccdc0a45278827f36f72aba5040eadefff129b869068797 \
-    --hash=sha256:8090b77f0791560b3c01263f6222006fe4c1d1d526539344afc4ecd9bd3e56f2 \
-    --hash=sha256:dbb98adb4281684eb54ce1f003b574bbc5768b9f614d7faa2c56f30e18519ec7 \
-    --hash=sha256:be8a929c6178bb6cbe9e2c858be62fa08966a39ae758a8493a88f0ed1efb6097
+protobuf==3.15.6 \
+    --hash=sha256:1771ef20e88759c4d81db213e89b7a1fc53937968e12af6603c658ee4bcbfa38 \
+    --hash=sha256:1a66261a402d05c8ad8c1fde8631837307bf8d7e7740a4f3941fc3277c2e1528 \
+    --hash=sha256:eac23a3e56175b710f3da9a9e8e2aa571891fbec60e0c5a06db1c7b1613b5cfd \
+    --hash=sha256:9ec220d90eda8bb7a7a1434a8aed4fe26d7e648c1a051c2885f3f5725b6aa71a \
+    --hash=sha256:88d8f21d1ac205eedb6dea943f8204ed08201b081dba2a966ab5612788b9bb1e \
+    --hash=sha256:eaada29bbf087dea7d8bce4d1d604fc768749e8809e9c295922accd7c8fce4d5 \
+    --hash=sha256:256c0b2e338c1f3228d3280707606fe5531fde85ab9d704cde6fdeb55112531f \
+    --hash=sha256:b9069e45b6e78412fba4a314ea38b4a478686060acf470d2b131b3a2c50484ec \
+    --hash=sha256:24f4697f57b8520c897a401b7f9a5ae45c369e22c572e305dfaf8053ecb49687 \
+    --hash=sha256:d9ed0955b794f1e5f367e27f8a8ff25501eabe34573f003f06639c366ca75f73 \
+    --hash=sha256:822ac7f87fc2fb9b24edd2db390538b60ef50256e421ca30d65250fad5a3d477 \
+    --hash=sha256:74ac159989e2b02d761188a2b6f4601ff5e494d9b9d863f5ad6e98e5e0c54328 \
+    --hash=sha256:30fe4249a364576f9594180589c3f9c4771952014b5f77f0372923fc7bafbbe2 \
+    --hash=sha256:45a91fc6f9aa86d3effdeda6751882b02de628519ba06d7160daffde0c889ff8 \
+    --hash=sha256:83c7c7534f050cb25383bb817159416601d1cc46c40bc5e851ec8bbddfc34a2f \
+    --hash=sha256:9ec20a6ded7d0888e767ad029dbb126e604e18db744ac0a428cf746e040ccecd \
+    --hash=sha256:0f2da2fcc4102b6c3b57f03c9d8d5e37c63f8bc74deaa6cb54e0cc4524a77247 \
+    --hash=sha256:70054ae1ce5dea7dec7357db931fcf487f40ea45b02cb719ee6af07eb1e906fb \
+    --hash=sha256:1655fc0ba7402560d749de13edbfca1ac45d1753d8f4e5292989f18f5a00c215 \
+    --hash=sha256:2b974519a2ae83aa1e31cff9018c70bbe0e303a46a598f982943c49ae1d4fcd3
 googleapis-common-protos==1.53.0 \
     --hash=sha256:f6d561ab8fb16b30020b940e2dd01cd80082f4762fa9f3ee670f4419b4b8dbd0 \
     --hash=sha256:a88ee8903aa0a81f6c3cec2d5cf62d3c8aa67c06439b0496b49048fb1854ebf4