renovate: automerge minor updates, make the install-test blocking

The two open Renovate PRs (#138 alpine 3.22 -> 3.24, #139 nginx 1.30.4 ->
1.31.4) never automerged because both are minor updates while the rule
only covered patch and digest. Sidecar images barely move in patch steps,
so patch-only would practically never fire.

Automerging a chart update publishes it, so the install-test can no
longer report failures without blocking: continue-on-error is gone and
issue #22's stabilization phase ends here. Majors stay manual.

renovate.json5

@@ -71,8 +71,13 @@
       enabled: false,
     },
     {
-      description: "Patch and digest updates merge themselves once the PR pipeline is green.",
-      matchUpdateTypes: ["patch", "digest"],
+      description: [
+        "Patch, minor and digest updates merge themselves once the PR pipeline is green.",
+        "Minor is included because the sidecar images barely move in patch steps —",
+        "alpine 3.22 -> 3.24 and nginx 1.30.4 -> 1.31.4 are both minor, so a patch-only",
+        "rule would practically never fire. Majors stay manual.",
+      ],
+      matchUpdateTypes: ["patch", "minor", "digest"],
       automerge: true,
       automergeType: "pr",
       // Renovate waits for the checks itself instead of handing the PR to