Sync GitHub actions with other repos (DSpace & dspace-angular)

[tdonohue]

This PR only updates GitHub Actions to add more automation to RestContract repo.

• Add ability to port PR between branches to allow us to version REST Contract (there is now a dspace-7_x branch for 7.x and a main for 8.x)
• Add label merge conflicts action to add merge conflict label to conflicting PRs
• Add action to assign PRs back to their creator (just makes creator easier to see on Project boards)
• Minor updates to existing action

No changes to the actual REST contract were made.

[tdonohue]

Added the necessary label to port these GitHub actions changes to dspace-7_x branch. This will be a test that these GitHub actions are working properly.

Merging immediately in order to test these actions are working.

[dspace-bot]

Successfully created backport PR for dspace-7_x:

• [Port dspace-7_x] Sync GitHub actions with other repos (DSpace & dspace-angular) #248