Improve updating behaviour

[MikeMcQuaid]

• Rename use of "preinstall" to "auto update". The original "preinstall" naming came from the fact that we used to only auto-update before brew install but now that it's many commands: this is more confusing than useful.
• Add HOMEBREW_NO_UPDATE_REPORT_ONLY_INSTALLED and remove HOMEBREW_UPDATE_REPORT_ONLY_INSTALLED; the latter is now the default and the prior provides an opt-out for better output, performance and avoiding reading potentially untrusted formulae.
• Add HOMEBREW_UPDATE_FORMULA_VERSION_CHECKS and don't check formula versions by default for better performance by default.

CC @boblail for thoughts.

[BrewTestBot]

Review period will end on 2022-05-19 at 19:58:52 UTC.

[samford]

Outside of the environment variable description mentioned above, this looks good to me. I tested out this branch and I like the configurability the new environment variables allow (they work as expected for my use case).

The installed-only brew update behavior does feel like a sensible default for the average user. As someone who runs brew update several times a day, I'm occasionally surprised at how long some users can go between updates. On the off chance that someone hasn't updated for a while and then runs brew update (instead of brew install or brew upgrade), the installed-only output should be considerably less overwhelming (especially with the increased output from version-changes-only filtering being disabled by default).

[BrewTestBot]

Review period ended.

[boblail]

LGTM!