update rubocop configuration for CI

[rei-moo]

Updates our rubocop config to prevent lint errors from blocking CI.

Once #560 is unblocked we can focus on Ruby 4.0 support and cleaning up the linting rules that were disabled in this PR.

All PRs:

• Has tests
• Documentation updated

[Copilot]

Pull request overview

This PR updates the Rubocop configuration to prevent lint errors from blocking CI until Ruby 4.0 support work and linting cleanup can be completed. The changes temporarily disable several style and lint cops while also updating a test to use public_send instead of send to address GitHub's custom linting rule.

• Disables five Rubocop cops that were causing CI failures across different Ruby versions
• Changes method invocation from send to public_send in a spec test for public attribute access
• Sets TargetRubyVersion to 2.6 to ensure consistent linting behavior

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
.rubocop.yml	Adds configuration to disable five cops (Style/ClassMethodsDefinitions, Style/OrAssignment, Layout/SpaceInsideHashLiteralBraces, Lint/ParenthesesAsGroupedExpression, Lint/RedundantCopDisableDirective) and sets TargetRubyVersion to 2.6
spec/lib/secure_headers/configuration_spec.rb	Updates test to use public_send instead of send for accessing public attributes, with inline cop disablement for GitHub/AvoidObjectSendWithDynamicMethod

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[Copilot]

Disabling the Lint/RedundantCopDisableDirective cop will hide instances where rubocop disable directives are no longer needed. This makes it harder to maintain clean code and remove outdated suppressions over time. Consider keeping this cop enabled to track when directives become redundant as rules change.

Suggested change

	Lint/RedundantCopDisableDirective:
	Enabled: false