E2E test: source code management data-testid migration

What does this MR do and why?

Resolves #415285 (closed)

Part of gitlab-org/quality/quality-engineering/team-tasks#1825 (closed)

Migrates data-qa-selectors to data-testid for the following SCM-related page object files used in the E2E tests:

• qa/qa/page/component/commit_modal.rb
• qa/qa/page/merge_request/show.rb
• qa/qa/page/project/branches/show.rb
• qa/qa/page/project/commit/show.rb

How to set up and validate locally

Since this affects many E2E tests, probably best to validate using pipeline results.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.