Update paths and build instructions for macOS builds

[jon-signal]

This updates our build instructions for creating Redis binaries for macOS. The two big things are:

1. Explicitly document that this assumes two parallel Homebrew installations (at least if the intent is to build both macOS binaries in a single shot); this has always been the case, but we can be clearer about it!
2. Update expected openssl paths; somewhere along the line, Homebrew changed where openssl 1.1 gets installed to avoid conflicts with openssl 3.