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/webhook.go

@@ -35,7 +35,7 @@ func loadWebhookFrom(rootCfg ConfigProvider) {
 	Webhook.DeliverTimeout = sec.Key("DELIVER_TIMEOUT").MustInt(5)
 	Webhook.SkipTLSVerify = sec.Key("SKIP_TLS_VERIFY").MustBool()
 
-	deprecatedSetting(rootCfg, "webhook", "ALLOWED_HOST_LIST", "security", "ALLOWED_HOST_LIST", "v28.0.0")
+	deprecatedSetting(rootCfg, "webhook", "ALLOWED_HOST_LIST", "security", "ALLOWED_HOST_LIST")
 	Webhook.AllowedHostList = sec.Key("ALLOWED_HOST_LIST").MustString(Security.AllowedHostList)
 	if Webhook.AllowedHostList != Security.AllowedHostList {
 		checkHostList("[webhook] ALLOWED_HOST_LIST", Webhook.AllowedHostList, false)