Upgrade runtime and packaging metadata to Ruby 3.1.x

[Copilot]

This updates the gem's declared runtime target to Ruby 3.1.x and aligns CI and published metadata with that support level. It also removes a packaging footgun surfaced during the upgrade path.

• Runtime target

  • Bump .ruby-version to 3.1.6
  • Raise required_ruby_version in the gemspec to >= 3.1.0

• CI alignment

  • Move the RSpec workflow container from Ruby 3.0.6 to 3.1.6

• Dependency / gemspec cleanup

  • Tighten virtus from an open-ended dependency to ~> 2.0
  • Drop the legacy # coding: utf-8 header

• Packaging cleanup

  • Exclude built .gem artifacts from spec.files
  • Ignore root-level .gem outputs in .gitignore
  • Remove the checked-in built gem artifact from the repository

• Docs

  • Refresh the README's Ruby compatibility and dependency examples to match the current runtime and Faraday stack

spec.required_ruby_version = ">= 3.1.0"
spec.add_dependency "virtus", "~> 2.0"