Update minitest requirement from ~> 5.0 to ~> 6.0 in /gem

[dependabot]

Updates the requirements on minitest to permit the latest version.

Changelog

Sourced from minitest's changelog.

=== 6.0.6 / 2026-04-30

• 2 bug fixes:

  • Fix using assert_equal/same/nil w/ BasicObject by comparing w/ nil == exp. (mtasaka)
  • Removed private Assertions#_where as it is no longer used.

=== 6.0.5 / 2026-04-20

• 2 bug fixes:

  • Avoid circular requires in lib/minitest/server_plugin.rb.
  • Raise TypeError if assert_raises is passed anything but modules/classes.

=== 6.0.4 / 2026-04-14

• 1 bug fix:

  • Fixed refute_predicate to call assert_respond_to w/ include_all:true like assert_predicate does. (jparker)

=== 6.0.3 / 2026-03-31

• 1 bug fix:

  • assert_same(nil, value) no longer allowed. Use assert_nil to be explicit. (paddor)

=== 6.0.2 / 2026-02-23

• 7 bug fixes:

  • Added missing drb dependency from minitest-server. (y-yagi)
  • Cleaned up bisect's sub-commands to load files directly, not ruby+require.
  • Fixed bisect to use same cmd ($0) to run tests as original invocation.
  • load server_plugin if server is loaded. (y-yagi)
  • Manipulate local args instead of ARGV in Sprint.run.
  • Only show --bisect in usage if using minitest.
  • Remove -Itest:lib from bisect's rb_flags since bin/minitest adds them.

=== 6.0.1 / 2025-12-26

• 1 minor enhancement:

  • Added new rake task test:fu to Minitest::TestTask, to only run tests with FU (focused units?) in their name. This should mostly obviate the need for the minitest-focus plugin.

• 5 bug fixes:

  • Fixed --help and --version exiting 1. (grosser)
  • Fixed method signature of Minitest::Benchmark.run. (flavorjones)

... (truncated)

Commits

• f6180b0 prepped for release
• 23bc7f2 - Removed private Assertions#_where as it is no longer used.
• c471347 - Fix using assert_equal/same/nil w/ BasicObject by comparing w/ nil == exp...
• 89c3e62 Branching minitest to version 6.0.5
• 6790f86 - Raise TypeError if assert_raises is passed anything but modules/classes.
• 235fa5b - Avoid circular requires in lib/minitest/server_plugin.rb.
• 5f0482e prepped for release
• b12f87f - Fixed refute_predicate to call assert_respond_to w/ include_all:true like a...
• 649b075 prepped for release
• a2d0904 - assert_same(nil, value) no longer allowed. Use assert_nil to be explicit. (...
• Additional commits viewable in compare view

[dependabot]

Looks like minitest is up-to-date now, so this is no longer needed.