Bump to 6.0.0 + refresh README header and compatibility

[serradura]

Summary

Major version bump to mark the Ruby floor change that landed in #74 (gemspec required_ruby_version raised from >= 2.1.0 to >= 2.7.0). Mirrors what was just shipped on u-attributes 3.0 so all three sibling gems stay structurally consistent.

Changes

• lib/kind/version.rb: 5.10.0 → 6.0.0.
• README header polished to mirror solid-process style:
  • h1 with the existing 🤷 emoji
  • 3-row badge layout: Gem Version + Build Status, Maintainability + Code Coverage, Ruby + Rails
  • Ruby/Rails badges switched to the dark gray palette (colorA=444 / colorB=333) used by solid-process
• Compatibility table updated:
  • 6.0.0 / v6.x is now the active line (ruby >= 2.7, activemodel >= 6.0)
  • 5.10.0 / v5.x row preserved with its real support range
  • 1.x..4.x rows dropped (long EOL)
• Added the Ruby × Rails support matrix ported from solid-process
• Documentation-versions table trimmed to unreleased + 6.0.0 + 5.10.0
• Installation example pins to ~> 6.0

Release checklist (once merged)

• bundle exec rake release to publish kind-6.0.0
• Cut a v6.x branch so the docs link in the README resolves
• Coordinate with u-attributes 3.0 (runtime dep kind, >= 4.0, < 6.0 — needs a bump to allow < 7.0 before the next u-attributes release)
• Same for u-case 5.0 (runtime dep on kind — verify and bump the constraint upper bound if needed)

Test plan

• Full Ruby 2.7..head / Rails 6.0..edge matrix stays green (no behavior changes in this PR, only version + README)

🤖 Generated with Claude Code