ci: make changelog-checker only validate PR number against main base (#10844)

.changelog/10844.txt

@@ -0,0 +1,3 @@
+```release-note:improvement
+ci: make changelog-checker only validate PR number against main base
+```

.github/workflows/changelog-checker.yml

@@ -23,13 +23,23 @@ jobs:
           fetch-depth: 0 # by default the checkout action doesn't checkout all branches
       - name: Check for changelog entry in diff
         run: |
+          pull_request_base_main=$(expr "${{ github.event.pull_request.base.ref }}" = "main")
+
           # check if there is a diff in the .changelog directory
-          changelog_files=$(git --no-pager diff --name-only HEAD "$(git merge-base HEAD "origin/${{ github.event.pull_request.base.ref }}")" -- .changelog/${{ github.event.pull_request.number }}.txt)
+          # for PRs against the main branch, the changelog file name should match the PR number
+          if [ pull_request_base_main ]; then
+            enforce_matching_pull_request_number="matching this PR number "
+            changelog_file_path=".changelog/${{ github.event.pull_request.number }}.txt"
+          else
+            changelog_file_path=".changelog/*.txt"
+          fi
+
+          changelog_files=$(git --no-pager diff --name-only HEAD "$(git merge-base HEAD "origin/main")" -- ${changelog_file_path})
 
           # If we do not find a file in .changelog/, we fail the check
           if [ -z "$changelog_files" ]; then
             # Fail status check when no .changelog entry was found on the PR
-            echo "Did not find a .changelog entry and the 'pr/no-changelog' label was not applied. Reference - https://github.com/hashicorp/consul/pull/8387"
+            echo "Did not find a .changelog entry ${enforce_matching_pull_request_number}and the 'pr/no-changelog' label was not applied. Reference - https://github.com/hashicorp/consul/pull/8387"
             exit 1
           else
             echo "Found .changelog entry in PR!"