Move cocoapods to gem

What does this MR do?

Moves cocoapods to be installed via ruby gem, following the official installation instructions.

Why was this MR needed?

The brew installation was not working:

tdout: |-
    ==> Testing cocoapods
    ==> /usr/local/Cellar/cocoapods/1.10.1/bin/pod list
    Last 15 lines from /Users/gitlab/Library/Logs/Homebrew/cocoapods/test.01.pod:
  
    /System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/lib/ruby/2.6.0/rubygems/dependency.rb:311:in `to_specs': Could not find 'CFPropertyList' (>= 2.3.3, < 4.0) among 150 total gem(s) (Gem::MissingSpecError)
    Checked in 'GEM_PATH=/usr/local/Homebrew/Library/Homebrew/vendor/bundle/ruby/2.6.0:/usr/local/Cellar/cocoapods/1.10.1/libexec', execute `gem env` for more information
            from /System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/lib/ruby/2.6.0/rubygems/specification.rb:1449:in `block in activate_dependencies'
            from /System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/lib/ruby/2.6.0/rubygems/specification.rb:1438:in `each'
            from /System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/lib/ruby/2.6.0/rubygems/specification.rb:1438:in `activate_dependencies'
            from /System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/lib/ruby/2.6.0/rubygems/specification.rb:1420:in `activate'
            from /System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/lib/ruby/2.6.0/rubygems/specification.rb:1452:in `block in activate_dependencies'
            from /System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/lib/ruby/2.6.0/rubygems/specification.rb:1438:in `each'
            from /System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/lib/ruby/2.6.0/rubygems/specification.rb:1438:in `activate_dependencies'
            from /System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/lib/ruby/2.6.0/rubygems/specification.rb:1420:in `activate'
            from /System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/lib/ruby/2.6.0/rubygems.rb:304:in `block in activate_bin_path'
            from /System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/lib/ruby/2.6.0/rubygems.rb:303:in `synchronize'
            from /System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/lib/ruby/2.6.0/rubygems.rb:303:in `activate_bin_path'
            from /usr/local/Cellar/cocoapods/1.10.1/libexec/bin/pod:23:in `<main>'

What's the best way to test this MR?

Launch a VM from this MR, and execute pod

scripts/dev-vm --base-image mr-76-toolchain-11.img new      

# ssh with address & port, base VM password in 1password
ssh -o Port=8822 gitlab@10.221.188.124

# in the VM run
pod

You should see:

gitlab@macos-shared-runner ~ % pod
Usage:

    $ pod COMMAND

      CocoaPods, the Cocoa library package manager.

Commands:

What are the relevant issue numbers?

Closes #67 (closed)