feat: Upgrade dependencies for Ruby 4.0

[aandreassa]

Upgrading to minitest 6, adding ostruct & updating other gems.

Added minitest-mock as it is now separate from minitest.

NOTE: minitest 5 will be the last in the minitest family to support
      ruby 1.8 to 2.7. If you need to keep using these versions,
      you need to pin your dependency to minitest with something
      like "~> 5.0". See History.rdoc to locate compatible
      versions.

Further, minitest 6 will be dropping the following:

      + MiniTest (it's been Minitest for >10 years)
      + MiniTest::Unit
      + MiniTest::Unit::TestCase
      + assert_send (unless you argue for it well)
      + assert_equal nil, obj
      + mocks and stubs: moving minitest/mock.rb to its own gem

[viacheslav-rostovtsev]

There's a file that doesnt belong otherwise lgtm

[viacheslav-rostovtsev]

I don't think this file should be here

[aandreassa]

Removed it, thanks!