Upgrade to Rails 7.2

[tungleduyxyz]

Upgrade the engine to Rails 7.2.

• Require rails ~> 7.2 in the gemspec
• Require Ruby >= 3.1.0 (minimum Ruby version for Rails 7.2)

See killbill/killbill-admin-ui#655 for the corresponding kaui engine upgrade.

JRuby CI fix (i18n / Fiber.current)

CI on jruby-9.4.2.0 failed during boot with:

NoMethodError: undefined method `current' for Fiber:Class
Did you mean?  __current__
  initialize at i18n-1.15.2/lib/i18n/config.rb:14

Root cause: i18n 1.15.x switched I18n.config to fiber-based storage. The original 1.15.0 report (ruby-i18n/i18n#735) covered the Fiber[:i18n_config] call breaking CRuby < 3.2; 1.15.1/1.15.2 reworked that but I18n::Config#initialize still calls Fiber.current. JRuby 9.4.2.0 doesn't expose a public Fiber.current (only the internal __current__; the public alias was added in a later 9.4.x release), so any i18n >= 1.15.0 fails at boot on that runtime.

This PR therefore also:

• Locks i18n to ~> 1.14.0 (1.14.x has no fiber code path, and satisfies activesupport 7.2's i18n >= 1.6, < 2) — matching the existing pins in kaui and kaui-standalone
• Bumps CI JRuby from 9.4.2.0 to 9.4.15.0 (which provides a proper Fiber.current) — matching kaui's CI