E2E test: replace MR haml qa-selectors with testids

What does this MR do and why?

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

Migrates remaining HAML data-qa-selectors to data-testid for MR pages in the E2E tests:

• qa/qa/ee/page/merge_request/show.rb
• qa/qa/page/merge_request/index.rb
• qa/qa/page/merge_request/show.rb

Since I had to update some shared components, this includes some updates to the following also:

• qa/qa/page/project/web_ide/vscode.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.