3.x: Release to Sonatype directly

[akarnokd]

Update the release process to use Sonatype directly via the help of the com.vanniktech:gradle-maven-publish-plugin.

In order for it to work, two secrets has to be set in the settings:

• SIGNING_PRIVATE_KEY
• SIGNING_PASSWORD

The SIGNING_PRIVATE_KEY should be the ascii-armored private key to sign the JAR files and can be generated from a pgp keychain:

gpg --export-secret-key --armor ___key_name_here___ > private_key.asc

The SIGNING_PASSWORD is then the password for that private_key.asc.

I tested the process on my local machine as much as I could, but we'll only know if it really works when doing an RxJava release.

Resolves #7179

[codecov]

Codecov Report

Merging #7181 (b2b4e33) into 3.x (0cb8978) will decrease coverage by 0.06%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                3.x    #7181      +/-   ##
============================================
- Coverage     99.56%   99.49%   -0.07%     
  Complexity     6751     6751              
============================================
  Files           747      747              
  Lines         47350    47350              
  Branches       6367     6367              
============================================
- Hits          47143    47113      -30     
- Misses           94      105      +11     
- Partials        113      132      +19     

Impacted Files	Coverage Δ	Complexity Δ
...l/operators/observable/ObservableFlatMapMaybe.java	88.02% <0.00%> (-4.93%)	2.00% <0.00%> (ø%)
...rnal/operators/flowable/FlowableFlatMapSingle.java	93.60% <0.00%> (-4.66%)	2.00% <0.00%> (ø%)
...ternal/operators/completable/CompletableMerge.java	97.29% <0.00%> (-2.71%)	2.00% <0.00%> (ø%)
...ernal/operators/flowable/FlowableFlatMapMaybe.java	94.30% <0.00%> (-2.60%)	2.00% <0.00%> (ø%)
...nternal/operators/observable/ObservableCreate.java	96.58% <0.00%> (-2.57%)	2.00% <0.00%> (ø%)
...reactivex/rxjava3/processors/PublishProcessor.java	98.16% <0.00%> (-1.84%)	41.00% <0.00%> (-1.00%)
...vable/ObservableFlatMapCompletableCompletable.java	98.48% <0.00%> (-1.52%)	3.00% <0.00%> (ø%)
...internal/operators/flowable/FlowableSwitchMap.java	98.58% <0.00%> (-1.42%)	3.00% <0.00%> (ø%)
...ctivex/rxjava3/internal/util/QueueDrainHelper.java	98.61% <0.00%> (-1.39%)	57.00% <0.00%> (-1.00%)
...operators/flowable/FlowableFlatMapCompletable.java	98.71% <0.00%> (-1.29%)	2.00% <0.00%> (ø%)
... and 9 more

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[mostroverkhov]

FYI default plugins shipped with gradle can do that without external dependencies - maven-publish and signing are enough.

Just apply corresponding plugins on modules that need to be released - their publishing and signing configurations are rather tiny

External config is

signing.keyId
signing.password
signing.secretKeyRingFile

ossrhUsername
ossrhPassword

May be worth considering to reduce future maintenance burden.