fix: switch to github: backend for google-java-format, use jsonc for markdownlint config

Signed-off-by: Gregor Zeitlinger <gregor.zeitlinger@grafana.com>

Author: zeitlinger

.markdownlint.json

@@ -0,0 +1,10 @@
+{
+  // Line length — pre-existing across docs, README, benchmarks
+  "MD013": false,
+  // Fenced code blocks without surrounding blank lines — pre-existing
+  "MD031": false,
+  // Inline HTML — needed for badge images in README
+  "MD033": false,
+  // Emphasis used as heading — pre-existing in docs/content
+  "MD036": false
+}

.markdownlint.jsonc

@@ -2,7 +2,7 @@
 "cargo:https://github.com/grafana/flint" = "branch:feat/flint-v2"
 "go:github.com/gohugoio/hugo" = "v0.157.0"
 "go:github.com/grafana/oats" = "0.6.1"
-"ubi:google/google-java-format" = "1.26.0"
+"github:google/google-java-format" = "1.26.0"
 actionlint = "1.7.10"
 editorconfig-checker = "v3.6.1"
 java = "temurin-25.0.2+10.0.LTS"