Deprecate Go extraction reconciliation feature flag and CODEQL_EXTRACTOR_GO_BUILD_TRACING for custom builds

[angelapwen]

This PR removes the CODEQL_EXTRACTOR_GO_BUILD_TRACING from all tests as well as another unused environment variable, CODEQL_ACTION_RECONCILE_TRACING. Users who previously had CODEQL_EXTRACTOR_GO_BUILD_TRACING set with custom Go builds will be shown a warning suggesting that they remove the environment variable as it no longer has effect.

Additionally, this PR deprecates the use of the Go extraction reconciliation feature flag so that its functionality is always on (ie. Go will always be treated as a traced language). A separate PR removing the flag from dotcom will be merged after this PR.

Merge / deployment checklist

• Confirm this change is backwards compatible with existing workflows.
• Confirm the readme has been updated if necessary.
• Confirm the changelog has been updated if necessary.

[henrymercer]

I'm not sure if the variable deprecation should be part of the changelog here or not 🤔

I think it would make sense to add a changelog note in the PR that removes the feature flag check similar to the one we used before starting rollout, saying that we have now rolled out this change. You could then mention the variable deprecation there too — it won't affect most users but the Actions changelog can be more detailed.

[angelapwen]

I'm not sure if the variable deprecation should be part of the changelog here or not 🤔

I think it would make sense to add a changelog note in the PR that removes the feature flag check similar to the one we used before starting rollout, saying that we have now rolled out this change. You could then mention the variable deprecation there too — it won't affect most users but the Actions changelog can be more detailed.

👍 I'll make a note to do so in that PR

[angelapwen]

I ended up finding an error in the Debug Artifacts Upload PR check because of this change — I hadn't added the actions/setup-go@v3 step in those PR checks so there wasn't any Go source code found for artifact upload. I didn't look into why it was previously passing, but as the PR checks are all passing now I think this is ready for re-review (also pending PM approval of the user-facing warning)

[henrymercer]

Very nice! Just a couple of comments around the docs.

[henrymercer]

LGTM, subject to @AlonaHlobina's approval of the warning message.

[angelapwen]

Ok — I resolved some merge conflicts, made some fixes to PR checks that were failing due to the new Go build command being added to build.sh, and re-ran possibly flaky tests. I think this PR is ready for review again — all the "expected" checks are ones that I have removed/replaced as of this change.

[henrymercer]

Looks good modulo the CHANGELOG.