brew doctor: unstable instead of fail, ignore proj@7 warnings

[scpeters]

Repeat of #626 for install_bottle jobs.

Needed to fix the following:

• https://build.osrfoundation.org/job/ignition_garden-install_bottle-homebrew-amd64/72/

#BEGIN SECTION: brew doctor analysis
+ brew missing
+ brew missing
+ brew list
+ grep '^sdl$'
+ brew doctor
Please note that these warnings are just used to help the Homebrew maintainers
with debugging if you file an issue. If everything you use Homebrew for is
working fine: please don't worry or file an issue; just ignore this. Thanks!

Warning: Some installed formulae are deprecated or disabled.
You should find replacements for the following formulae:
  proj@7
Build step 'Execute shell' marked build as failure

[scpeters]

testing with this branch https://build.osrfoundation.org/job/ignition_garden-install_bottle-homebrew-amd64/74/

[scpeters]

added a change to address a critical build farm issue: mark brew doctor complaints as unstable instead of failures

testing: https://build.osrfoundation.org/view/os_homebrew/job/ignition_cmake2-install_bottle-homebrew-amd64/585/

[nuclearsandwich]

This looks okay. I asked one non-blocking question about running brew doctor while also suppressing known issues.

[nuclearsandwich]

Since brew doctor is now a soft failure rather than a hard one does it make sense to run it unconditionally and live with with the warning or otherwise let it warn about other issues while suppressing the proj complaint?

[scpeters]

I'd prefer to suppress the proj@7 complaint since we don't have any control over that; that's an issue with homebrew-core. brew doctor usually complains about out-of-date Xcode version on our build machines, which I like to be notified about.