[hotfix] Update CI strategy. This closes #61

1. Test all PRs for `main` against all supported versions, meaning 1.17.x and 1.18.x. That's because only PRs run the dependency convergence check, and not nightly builds.
2. Make sure that we test nightlies against all supported versions (currently 1.17.x for the `v3.0` branch plus 1.17.x and 1.18.x against `main`)
diff --git a/.github/workflows/push_pr.yml b/.github/workflows/push_pr.yml
index 3e54984..be8e327 100644
--- a/.github/workflows/push_pr.yml
+++ b/.github/workflows/push_pr.yml
@@ -25,7 +25,7 @@
   compile_and_test:
     strategy:
       matrix:
-        flink: [ 1.17.1 ]
+        flink: [ 1.17.1, 1.18-SNAPSHOT ]
     uses: apache/flink-connector-shared-utils/.github/workflows/ci.yml@ci_utils
     with:
       flink_version: ${{ matrix.flink }}
diff --git a/.github/workflows/weekly.yml b/.github/workflows/weekly.yml
index b4daceb..a9de998 100644
--- a/.github/workflows/weekly.yml
+++ b/.github/workflows/weekly.yml
@@ -26,8 +26,18 @@
     if: github.repository_owner == 'apache'
     strategy:
       matrix:
-        flink: [1.17-SNAPSHOT, 1.18-SNAPSHOT]
+        flink_branches: [{
+          flink: 1.17.1,
+          branch: v3.0
+        }, {
+          flink: 1.17.1,
+          branch: main
+        }, {
+          flink: 1.18-SNAPSHOT,
+          branch: main
+        }]
     uses: apache/flink-connector-shared-utils/.github/workflows/ci.yml@ci_utils
     with:
-      flink_version: ${{ matrix.flink }}
+      flink_version: ${{ matrix.flink_branches.flink }}
+      connector_branch: ${{ matrix.flink_branches.branch }}
       run_dependency_convergence: false