Remove the legacy mixins linting script and associated code.

Apparently if we have _no_ legacy util in our code base, tailwind prints
a warning which needs to be filtered out by our static analysis script.

The filtering can be removed once we don't compile the legacy utils
anymore (which we currently do for linting purposes).

This change installs a new gem called `async` to utilize fibers and
non-blocking IO features of Ruby 3.

We will use the `Async` DSL to utilize fibers to concurrently read
intermediary vulnerability export files while producing the final
report.

This fixes a spurious warning message:

```
Type application/netcdf is already registered as a variant of
application/netcdf.
```

mime-types diff: https://my.diffend.io/gems/mime-types/3.4.1/3.5.1
History: https://github.com/mime-types/ruby-mime-types/blob/main/History.md

mime-types-data diff:
https://my.diffend.io/gems/mime-types-data/3.2022.0105/3.2023.1003
History:
https://github.com/mime-types/mime-types-data/blob/main/History.md

Relates to https://gitlab.com/gitlab-org/gitlab/-/issues/369364

Changelog: fixed

- See ee/lib/remote_development/README.md for context
- Adds Railway Oriented Programming (ROP) to
  Remote Development Workspace Update module
- Add generic Result type (not domain specific)
  to support ROP

We can remove it after we drop Ruby 2 support.

Since the mail gem now requires net-protocol, there's no use
in adding an exception in Gemfile. We now ignore the warnings
in Ruby 2.7.

When scripts/static-analysis sees a warning it prints it on stderr
along with the following banner:

**** bin/rake config_lint had the following warning(s):

Since we are filtering certain warnings via `fail_on_warnings` we need
to filter this line as well.

This commit also turns all allowed warnings into regexp.

Usage:
  fail_on_warnings "some command" || exit 1
  run_timed_command "fail_on_warnings some command"

Example:
  $ run_timed_command "fail_on_warnings ls invalid"
  $ fail_on_warnings ls invalid
  There were warnings:
  ls: cannot access 'invalid': No such file or directory
  ==> 'fail_on_warnings ls invalid' failed (1) in 0 seconds.