chore: make DefaultTitleSource default to auto to match GitHub (#37767)

It is a changed (breaking) behavior introduced in 1.26, no need to have
such a breaking change.

custom/conf/app.example.ini

@@ -1172,7 +1172,7 @@ LEVEL = Info
 ;; Default source for the pull request title when opening a new PR.
 ;; "first-commit" uses the oldest commit's summary.
 ;; "auto" uses commit's summary if the PR only has one commit, normalizes the branch name if multiple commits.
-;DEFAULT_TITLE_SOURCE = first-commit
+;DEFAULT_TITLE_SOURCE = auto
 ;;
 ;; Delay mergeable check until page view or API access, for pull requests that have not been updated in the specified days when their base branches get updated.
 ;; Use "-1" to always check all pull requests (old behavior). Use "0" to always delay the checks.