fix(renovate): lowercase commit subject to satisfy commitlint (#937)

Renovate's packageRules combine a semantic commitMessagePrefix (e.g.
"chore(docker):") with a capitalized groupName (e.g. "Docker images"),
producing commit subjects like "chore(docker): Update Docker images".
commitlint's subject-case rule rejects the capitalized subject that
follows the prefix, failing the commitlint CI job on every PR raised
by this packageRule (e.g. PR #924).

commitMessageLowerCase: "auto" makes Renovate lowercase the first word
of the subject after the semantic prefix, fixing this for the Docker
group and any other capitalized groupName (GitHub Actions, E2E Docker
images) without changing how groups are displayed.

Co-authored-by: Guillaume Leccese <guillaume.leccese@theodo.com>

Author: DjinnS

renovate.json

@@ -9,6 +9,7 @@
   "ignorePresets": [
     ":semanticPrefixFixDepsChoreOthers"
   ],
+  "commitMessageLowerCase": "auto",
   "packageRules": [
     {
       "matchManagers": [