[llvm] [Workflows] Enable commit access requests via GitHub issues (PR #100458)

[Tom Stellard via llvm-commits]

tstellar wrote:

I think I've addressed the most of the comments.  Based on the discussion about whether or not to ask for a justification for commit access, would it make sense to first land a separate PR, that brings the documentation up to date with reality. e.g.
 ```
s/We grant commit access to contributors with a track record of submitting high
quality patches./We grant commit access to contributors with a valid justification/
```

https://github.com/llvm/llvm-project/pull/100458