renovate.json: replace `CI:` prefix with `GHA:`

All bumped dependencies are in GHA.

Follow-up to 6225d7ba2f7dcad322776fc1cadae63e530de705 #19547
diff --git a/renovate.json b/renovate.json
index 616fd1a..3342181 100644
--- a/renovate.json
+++ b/renovate.json
@@ -21,7 +21,7 @@
         "pinDigest",
         "digest"
       ],
-      "commitMessagePrefix": "CI: ",
+      "commitMessagePrefix": "GHA: ",
       "labels": [
         "CI"
       ]
@@ -30,7 +30,7 @@
       "matchManagers": [
         "custom.regex"
       ],
-      "commitMessagePrefix": "CI: ",
+      "commitMessagePrefix": "GHA: ",
       "labels": [
         "CI"
       ]