Update PHP to 7.4 macOS Catalina with brew

You can find my similar answer for php@7.2.

• brew install php@7.4
• brew link --force --overwrite php@7.4
• brew services start php@7.4
• export PATH="/usr/local/opt/php@7.4/bin:$PATH"
• export PATH="/usr/local/opt/php@7.4/sbin:$PATH"