Revert "feat(ci): switch to our custom runners (default)"

This reverts commit aa15b52.

Turns out that the project is public and we don't share our custom runners with
public repos.

Until we find out why this is public we better revert this change.

Author: potyl

.github/workflows/merge-main.yaml

@@ -10,7 +10,7 @@ on:
 jobs:
   build-push:
     name: Build and push application image
-    runs-on: default
+    runs-on: ubuntu-latest
     permissions:
       id-token: write
       contents: read

.github/workflows/release-tag.yaml

@@ -10,7 +10,7 @@ jobs:
 
   re-tag:
     name: Re-tag image
-    runs-on: default
+    runs-on: ubuntu-latest
     permissions:
       id-token: write
       contents: read