PR changing maintainers should notify original maintainers only

[haampie]

spack/spack#25156 (comment)

• The list of maintainers should be queried from the develop branch, not the PR itself.
• When there were no maintainers previously and the PR adds new maintainers, the bot shouldn't comment at all.

[adamjstewart]

I disagree, the PR should ping both new and old maintainers. Consider this situation. Someone adds the developer of the software library as a maintainer of the recipe without their consent. We want them to know about this so they aren't being notified out of the blue on future PRs.

[alecbcs]

I think I agree with @adamjstewart it'd be jarring to have someone add you to a package as maintainer without your acknowledgment.

@haampie do you agree with this assessment or are we overlooking something?