Modernize packaging, fix bugs, bump to 2.0.0

Bug fixes:
- HEAD vs GET: accessibility check now uses HTTPI.request(:head, ...)
  to match documented behavior. New :http_method option (default :head).
- allow_blank: false is now honored (previously short-circuited blanks).
- Rescue branch no longer calls url_format_valid? on a half-mutated URI;
  invalid_url is added unconditionally and the method returns.
- Test suite now uses webmock; no live network calls.

Packaging:
- Hand-written gemspec replacing juwelier. MIT SPDX, rubygems_mfa_required,
  modern metadata uris, git ls-files. Required Ruby >= 3.1.
- Drop activerecord dep in favor of activemodel >= 6.1.
- Move version to lib/url_validation/version.rb.
- Replace .travis.yml with GitHub Actions matrix (Ruby 3.1-3.4 x AM 7-8)
  plus standardrb lint job.
- Add CHANGELOG, bin/console, bin/setup, .standard.yml.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Version bump to 1.2.0

Version 1.1.0