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>

templates/swagger/v1-openapi3.generated.json

@@ -6167,6 +6167,11 @@
             "type": "string",
             "x-go-name": "ProjectsMode"
           },
+          "squash_merge_commit_message": {
+            "description": "set the default squash merge commit message body: \"PR_BODY\", \"COMMIT_MESSAGES\", or \"BLANK\".",
+            "type": "string",
+            "x-go-name": "SquashMergeCommitMessage"
+          },
           "template": {
             "description": "either `true` to make this repository a template or `false` to make it a normal repository",
             "type": "boolean",
@@ -9876,6 +9881,10 @@
             "type": "integer",
             "x-go-name": "Size"
           },
+          "squash_merge_commit_message": {
+            "type": "string",
+            "x-go-name": "SquashMergeCommitMessage"
+          },
           "ssh_url": {
             "format": "uri",
             "type": "string",

templates/swagger/v1-swagger.generated.json

@@ -29273,6 +29273,11 @@
           "type": "string",
           "x-go-name": "ProjectsMode"
         },
+        "squash_merge_commit_message": {
+          "description": "set the default squash merge commit message body: \"PR_BODY\", \"COMMIT_MESSAGES\", or \"BLANK\".",
+          "type": "string",
+          "x-go-name": "SquashMergeCommitMessage"
+        },
         "template": {
           "description": "either `true` to make this repository a template or `false` to make it a normal repository",
           "type": "boolean",
@@ -32962,6 +32967,10 @@
           "format": "int64",
           "x-go-name": "Size"
         },
+        "squash_merge_commit_message": {
+          "type": "string",
+          "x-go-name": "SquashMergeCommitMessage"
+        },
         "ssh_url": {
           "type": "string",
           "x-go-name": "SSHURL"