fix(renovate): switch to regexManagers format (pre-v37 compat)

Renovate on this Gitea instance runs a pre-v37 version that does not
support the new customManagers format. Switch to the legacy regexManagers
format with fileMatch for compatibility.

- customManagers → regexManagers
- managerFilePatterns → fileMatch
- Remove customType (not supported in legacy format)

renovate.json

@@ -1,26 +1,23 @@
 {
   "$schema": "https://docs.renovatebot.com/renovate-schema.json",
-  "customManagers": [
+  "regexManagers": [
     {
-      "customType": "regex",
       "description": "Detect SABnzbd release version pin",
-      "managerFilePatterns": ["Dockerfile"],
+      "fileMatch": ["Dockerfile"],
       "matchStrings": ["SABNZBD_VERSION=(?<currentValue>[0-9.]+)"],
       "depNameTemplate": "sabnzbd/sabnzbd",
       "datasourceTemplate": "github-releases"
     },
     {
-      "customType": "regex",
       "description": "Detect Tini release version pin",
-      "managerFilePatterns": ["Dockerfile"],
+      "fileMatch": ["Dockerfile"],
       "matchStrings": ["\"TINI_VERSION\"\\s+\"v?(?<currentValue>[0-9.]+)\""],
       "depNameTemplate": "krallin/tini",
       "datasourceTemplate": "github-releases"
     },
     {
-      "customType": "regex",
       "description": "Detect par2cmdline release version pin",
-      "managerFilePatterns": ["Dockerfile"],
+      "fileMatch": ["Dockerfile"],
       "matchStrings": ["PAR2CMDLINE_VERSION=v?(?<currentValue>[0-9a-z.]+-mt\\d+)"],
       "depNameTemplate": "jkansanen/par2cmdline-mt",
       "datasourceTemplate": "github-tags"