Apply my workarounds for Sonoma 14.6+

[micolous]

I propose changing this Homebrew formula to apply the workarounds I've proposed here to address this once and for all:

• Create a symlink to ssh-askpass (or install it directly) at /private/var/select/X11/bin/ssh-askpass.

  Because this is outside HOMEBREW_PREFIX (and Homebrew doesn't like that), I think that this Formula will need to become a Cask.

• Add a dependency on the xquartz cask, so that something sets the DISPLAY environment variable with SecureSocketWithKey.

  While you could replace this with a dummy LaunchAgent that sets DISPLAY in the same way, a non-functional X server at DISPLAY could break users.

• Change the caveat note to instruct users to log out and log in again to pick up XQuartz environment variable changes.

• Consider removing the ssh-agent.plist LaunchAgent entirely (or at least, note that it may not be needed in caveats).

  This should not be needed if DISPLAY is set and ssh-askpass can be found at /usr/X11R6/bin/ssh-askpass, but it may break someone with custom SSH binaries if they've patched it to check different paths.

This should make this formula work out-of-the-box with Apple's default ssh-agent config on 14.6 and later. It should also work fine with 14.5 and earlier.

Third-party/custom SSH binaries and configs should also work with this, provided they also check /usr/X11R6/bin/ssh-askpass (which is the default path).