[stable-2.16] Update ansible-core branch indicator file

[felixfontein]

Right now the stable-2.16 docs are built with ansible-core devel since the branch indicator file hasn't been updated after branching.

[felixfontein]

@oraNod @samccann this should probably be mentioned for a "create new branch for a corresponding ansible-core branch" checklist, if such a thing exists.

[gotmax23]

Thanks!

[samccann]

@felixfontein I don't have 'create a new branch' yet in the checklist.. I tend to create said checklists after...the branch is created :-)

That said, I did add this tidbit to verify in the core release checklist. What are the steps for pulling a new branch and I can add that as well

[felixfontein]

About creating the branch: maybe we have a document (or wiki? I think I prefer document in the repo) with instructions for such operations, because we'll likely forget about this again until in 6 months. That document could also contain checklist templates for other things, like a new core release.

About your release core checklist: I adjusted the text a bit (to make it clear that has to be done in the stable-2.16 branch) and checked it ;-)