Add libssh2 to Brewfile

What does this merge request do and why?

Rugged/libgit2 needs libssh2 to work. We've had two people report that Rugged no longer loaded after they migrated to an Apple M1:

Generating gitlab-org/gitlab Rails translations
--------------------------------------------------------------------------------
rake aborted!
LoadError: dlopen(/Users/mikolajwawrzyniak/.asdf/installs/ruby/2.7.5/lib/ruby/gems/2.7.0/gems/rugged-1.2.0/lib/rugged/rugged.bundle, 0x0009): symbol not found in flat namespace (_libssh2_agent_connect) - /Users/mikolajwawrzyniak/.asdf/installs/ruby/2.7.5/lib/ruby/gems/2.7.0/gems/rugged-1.2.0/lib/rugged/rugged.bundle
/Users/mikolajwawrzyniak/gitlab/gitlab-development-kit/gitlab/config/application.rb:15:in `<top (required)>'
/Users/mikolajwawrzyniak/gitlab/gitlab-development-kit/gitlab/Rakefile:9:in `require'
/Users/mikolajwawrzyniak/gitlab/gitlab-development-kit/gitlab/Rakefile:9:in `<top (required)>'
/Users/mikolajwawrzyniak/.asdf/installs/ruby/2.7.5/bin/bundle:25:in `load'
/Users/mikolajwawrzyniak/.asdf/installs/ruby/2.7.5/bin/bundle:25:in `<main>'

Caused by:
LoadError: cannot load such file -- rugged/2.7/rugged
/Users/mikolajwawrzyniak/gitlab/gitlab-development-kit/gitlab/config/application.rb:15:in `<top (required)>'
/Users/mikolajwawrzyniak/gitlab/gitlab-development-kit/gitlab/Rakefile:9:in `require'
/Users/mikolajwawrzyniak/gitlab/gitlab-development-kit/gitlab/Rakefile:9:in `<top (required)>'
/Users/mikolajwawrzyniak/.asdf/installs/ruby/2.7.5/bin/bundle:25:in `load'
/Users/mikolajwawrzyniak/.asdf/installs/ruby/2.7.5/bin/bundle:25:in `<main>'
(See full trace by running task with --trace)
make: *** [.gitlab-translations] Error 1
❌️ ERROR: Failed to install.

Impacted categories

The following categories relate to this merge request:

• gdk-reliability - e.g. When a GDK action fails to complete.
• gdk-usability - e.g. Improvements or suggestions around how the GDK functions.
• gdk-performance - e.g. When a GDK action is slow or times out.

Merge request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise an issue to follow-up.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.