enhance(pull)!: add per-repository default squash merge commit message (#39816)

Adds a repository setting for the default squash merge commit message
body with GitHub's options `PR_BODY`, `COMMIT_MESSAGES` and `BLANK`,
also settable via `squash_merge_commit_message` in the API. The commit
title stays the pull request title.

1. Breaking: API squash merges without `merge_message_field` now get the
same default message as the web merge box instead of a title-only
commit, which affects clients sending only `Do` like Renovate. An empty
value keeps title-only.
2. Breaking: `[repository.pull-request] DEFAULT_SQUASH_MERGE_MESSAGE`
replaces `POPULATE_SQUASH_COMMENT_WITH_COMMIT_MESSAGES`, which still
works as a deprecated fallback. Repositories store the default when
created or next saved.
3. Deprecated app.ini keys now migrate through one table instead of
per-loader fallbacks, which also fixes the ignored `[mailer] CERT_FILE`
and `KEY_FILE` fallbacks. Deprecation warnings no longer claim a removal
version.

<img width="1891" height="571" alt="grafik"
src="https://github.com/user-attachments/assets/0fdcde13-1f11-4156-b598-d80eb0d04cbe"
/>

---------

Co-authored-by: silverwind <me@silverwind.io>

modules/setting/oauth2.go

@@ -122,12 +122,6 @@ func loadOAuth2From(rootCfg ConfigProvider) {
 		OAuth2.DefaultApplications = nil
 	}
 
-	// Handle the rename of ENABLE to ENABLED
-	deprecatedSetting(rootCfg, "oauth2", "ENABLE", "oauth2", "ENABLED", "v1.23.0")
-	if sec.HasKey("ENABLE") && !sec.HasKey("ENABLED") {
-		OAuth2.Enabled = sec.Key("ENABLE").MustBool(OAuth2.Enabled)
-	}
-
 	if !filepath.IsAbs(OAuth2.JWTSigningPrivateKeyFile) {
 		OAuth2.JWTSigningPrivateKeyFile = filepath.Join(AppDataPath, OAuth2.JWTSigningPrivateKeyFile)
 	}