Audit and update buttons on Projects::PipelinesController#index

What does this MR do?

• Updates the Run Pipeline button to use the confirm variant.
• Updates the buttons to use sentence-case instead of titlecase.
• Reorders the buttons to Clear runner caches, CI lint, Run pipeline.
• Updates related tests.
• Updates any reference to the "Run pipline" button in the docs to use the updated case.

This doesn’t change the casing for "Run Pipeline" when used as a proper noun, for example when referencing the Run Pipeline page.

Screenshots

Before	After

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because _____.
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Related to #273293 (closed)