[FEA] Automerge to support new branch strategy

[pxLi]

Is your feature request related to a problem? Please describe.
sub-item of #35 for automerge action

Describe the solution you'd like
Auto merge from release/YY.MM to main:

1. Detect PR of release/YY.MM got merged
2. Check if any release tag exists for vYY.MM.0
3. If it exists, then skip. If not, continue 4.
4. Create PR from release/YY.MM to main, and try auto-merge as create a merge commit

[pxLi]

also ref: #6