tests: remove mentions of ALLOW_*_COMPATIBILITY_BREAKAGE (#11618)

## Problem

There are mentions of `ALLOW_BACKWARD_COMPATIBILITY_BREAKAGE` and
`ALLOW_FORWARD_COMPATIBILITY_BREAKAGE`, but in reality, this mechanism
doesn't work, so let's remove it to avoid confusion.

The idea behind it was to allow some breaking changes by adding a
special label to a PR that would `xfail` the test. However, in practice,
this means we would need to carry this label through all subsequent PRs
until the release (and artifact regeneration). This approach isn't
really viable, as it increases the risk of missing a compatibility break
in another PR.

## Summary of changes
- Remove mentions and handling of
`ALLOW_BACKWARD_COMPATIBILITY_BREAKAGE` /
`ALLOW_FORWARD_COMPATIBILITY_BREAKAGE`

.github/actions/run-python-test-set/action.yml

@@ -113,8 +113,6 @@ runs:
         TEST_OUTPUT: /tmp/test_output
         BUILD_TYPE: ${{ inputs.build_type }}
         COMPATIBILITY_SNAPSHOT_DIR: /tmp/compatibility_snapshot_pg${{ inputs.pg_version }}
-        ALLOW_BACKWARD_COMPATIBILITY_BREAKAGE: contains(github.event.pull_request.labels.*.name, 'backward compatibility breakage')
-        ALLOW_FORWARD_COMPATIBILITY_BREAKAGE: contains(github.event.pull_request.labels.*.name, 'forward compatibility breakage')
         RERUN_FAILED: ${{ inputs.rerun_failed }}
         PG_VERSION: ${{ inputs.pg_version }}
         SANITIZERS: ${{ inputs.sanitizers }}