Update actions/checkout in GitHub Actions to v3

[martinmoene]

See expected-lite, PR45.

Progress:

• lest
• active-lite
• any-lite
• atomic-lite (WIP)
• bit-lite
• boolean-lite
• byte-lite
• expected-lite
• invoke-lite
• jthread-lite
• observer-ptr-lite
• optional-bare
• optional-fun-lite (no tests)
• optional-lite
• ring-span-lite
• scope-lite
• span-lite
• status-value-lite
• string-lite (WIP)
• string-view-lite
• type-lite
• value-ptr-lite
• variant-lite

[striezel]

This issue is a duplicate of #70, and most (all?) of the other projects have been handled there, so this issue here can be closed.

[martinmoene]

't has been a while :)

thank you!