ci: add PR labeler workflow for main/master branches (#7304)

Adds actions/labeler@v5 to automatically label pull requests:
- PRs targeting main get the 'flyte2' label
- PRs targeting master get the 'flyte' label

Signed-off-by: Kevin Su <pingsutw@apache.org>

Author: pingsutw

.github/labeler.yml

@@ -0,0 +1,5 @@
+flyte2:
+  - base-branch: ['^main$']
+
+flyte:
+  - base-branch: ['^master$']

.github/workflows/labeler.yml

@@ -0,0 +1,18 @@
+name: "Pull Request Labeler"
+
+on:
+  pull_request_target:
+    types: [opened, reopened, synchronize]
+
+permissions:
+  contents: read
+  pull-requests: write
+
+jobs:
+  label:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/labeler@v5
+        with:
+          configuration-path: .github/labeler.yml
+          sync-labels: false