[llvm] Changed Regex to accept /branch: and /cherry-pick: commands (PR #81002)

[via llvm-commits]

llvmbot wrote:

<!--IGNORE-->
>Taken account of these strings : 
>
>```
>"/cherry-pick: Hola mundo"
>"/branch: owner/repo/branch"
>"/cherry-pick Hola mundo"
>"/branch owner/repo/branch"
>"/branch : owner/repo/branch"
>"/cherry-pick : Hola mundo"
>```

Error: Command failed due to missing milestone.

https://github.com/llvm/llvm-project/pull/81002