walproposer: remove support for wp<->sk protocol v2

[arpad-m]

Remove support for protocol version 2 between the walproposer and safekeeper.

Builds on #12020
Part of #10326

[github-actions]

If this PR added a GUC in the Postgres fork or neon extension,
please regenerate the Postgres settings in the cloud repo:

make NEON_WORKDIR=path/to/neon/checkout \
  -C goapp/internal/shareddomain/postgres generate

If you're an external contributor, a Neon employee will assist in
making sure this step is done.

[arpad-m]

Filing as WIP because I think it would be a good idea to keep support for now in order to test protocol v2 support in the safekeepers, i.e. as a thing for pytest.

[github-actions]

8481 tests run: 7932 passed, 0 failed, 549 skipped (full report)

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Code coverage* (full report)

• functions: 32.3% (9049 of 27992 functions)
• lines: 48.5% (79444 of 163636 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
2ff680a at 2025-05-27T12:49:33.397Z :recycle:}