ci: make changelog-checker only validate PR number against main base

[mikemorris]

Intent of this change is to avoid spurious changelog-checker failures on manual backport PRs, where cherry-picking the changelog entry from the original commit will intentionally not match the PR number of the backport (matching the original change instead).

My bash might need some revisions to make sure the logic is correct and syntax is valid.

[mikemorris]

@alvin-huang Any idea why this check is now just failing with exit code 1 instead of failing and commenting on the PR? I didn't change much, but is there a different way these should be written now (like with https://github.com/actions/github-script#comment-on-an-issue?) or is this an effect of editing this file in the PR?

Edit: Check was failing on invalid bash syntax which is now fixed, PR comment had been intentionally removed in #9781 to prefer just using a status check instead.

[hc-github-team-consul-core]

🍒 If backport labels were added before merging, cherry-picking will start automatically.

To retroactively trigger a backport after merging, add backport labels and re-run https://circleci.com/gh/hashicorp/consul/428417.

[hc-github-team-consul-core]

🍒✅ Cherry pick of commit 2a98ed5 onto release/1.10.x succeeded!