bump v6.0.0

[Fivell]

Summary

Cuts the v6.0.0 release. Bumps lib/active_admin_import/version.rb from 5.1.0 → 6.0.0 and adds the changelog entry covering everything that landed since v5.1.0, plus a backfilled v5.1.0 entry that was missing from CHANGELOG.md.

Why 6.0.0 (and not 5.2.0)

Several breaking changes have accumulated:

• Dropped EOL versions — Ruby < 3.1, Rails < 7.1, ActiveAdmin < 3.0, and activerecord-import < 2.0 are no longer supported
• activerecord-import floor bumped to >= 2.0 (Bump activerecord-import floor to >= 2.0 #210)
• Action block invocation changed from instance_eval(&block) to instance_exec(result, options, &block) (Allow custom ImportResult subclass via :result_class option #214). Zero-arity blocks still work because Ruby procs ignore extra arguments — but anyone declaring do |x| previously would now receive result instead of nothing

That's enough to warrant a major.

Changelog highlights for 6.0.0

• Drop EOL Ruby/Rails/AA versions
• Rails 8.0 and ActiveAdmin 3.4 / 3.5.1 added to CI
• MySQL 8.0 + PostgreSQL 16 jobs added alongside the SQLite matrix (Add MySQL and PostgreSQL to CI database matrix #213)
• Switch JavaScript driver from Poltergeist to Cuprite
• New active_admin_import_context controller convention (Add active_admin_import_context convention for controller-provided context #211)
• New :result_class option for plugging in a custom ImportResult subclass (Allow custom ImportResult subclass via :result_class option #214)
• File caching / format-validation bug fixes (Fix bugs with caching and format validation when importing files #204)
• Documentation moved from the (long-stale) wiki and gh-pages site into the README (Move documentation from wiki/gh-pages into README #215)
• Coveralls badge replaced with a self-hosted shields.io endpoint badge served from Pages (Replace Coveralls badge with self-hosted coverage badge on Pages #216)

Test plan

• lib/active_admin_import/version.rb reads 6.0.0
• CHANGELOG.md has entries for both 6.0.0 and the previously-undocumented 5.1.0
• Compare links at the bottom of the changelog updated
• CI green
• After merge: tag v6.0.0 and push to rubygems