fix: attempt to fix Heimdall version tracking by Renovate

Alas over time the versioning schema of image tags by
`lscr.io/linuxserver/heimdall` changed several times, ranging from
simply incrementing a counter to a ISO 8601 date-alike variant to
finally something which comes close the semantic versioning.

With the default rule set, Renovate gets fooled by the ISO 8601 date
variant which was used in year 2021. The idea is to use a regular
expression for better matching which kicks out 2021 using a negative
lookahead and also require a minor version number in order not to get
caught by the counter-like increments.

As a test, the image tag is intentionally set to `2.6.0` while `2.6.1`
would be the latest release. This is just to verify whether Renovate
does to right thing now.

docker-compose/heimdall/docker-compose.yaml

@@ -1,7 +1,7 @@
 ---
 services:
   heimdall:
-    image: lscr.io/linuxserver/heimdall:2021.11.28
+    image: lscr.io/linuxserver/heimdall:2.6.0
     container_name: heimdall
     environment:
       - PUID=1000

renovate.json

@@ -55,6 +55,16 @@
       "packagePatterns": [
         "^([^/]+\\/)*mariadb(:.+)?$"
       ]
+    },
+    {
+      "managers": [
+        "docker-compose",
+        "dockerfile"
+      ],
+      "packagePatterns": [
+        "^([^/]+\\/)*heimdall(:.+)?$"
+      ],
+      "versioning": "regex:^(?<major>(?!2021)\\d+)\\.(?<minor>\\d+)(\\.(?<patch>\\d+))?$"
     }
   ],
   "separateMinorPatch": true