[mapserver-dev] updated Backport workflow

[Jeff McKenna]

Hi Devs,

Today I spent some effort on configuring a new Backport process, 
leveraging the same tool that is now relied upon heavily by several 
other projects such as QGIS, etc.  The big change now is that the 
backport bot is triggered by setting one of the "backport ..." *LABELS*. 
  Here is an example workflow:

  - create PR to master
  - then add one of the new "backport..." labels such as "backport 
branch-7-6" to the PR
  - when merged the bot will automagically create another PR to that branch

Here is an example PR, notice the label set in right panel: 
https://github.com/mapserver/mapserver/pull/6035

..which when merged magically generated a PR into branch-7-6: 
https://github.com/mapserver/mapserver/pull/6036

Let me know if anything needs modifying in the process, which is fully 
configured in the file: 
https://github.com/mapserver/mapserver/blob/master/.github/workflows/backport.yml

Give it a try and let me know,

-jeff