Remove Solargraph warnings

What does this MR do and why?

Disable the require_not_found reporter in Solargraph. This reporter stumbles over RSpec-specific setup. The developer recommends this procedure to get rid of the warnings.

Expected impact & dry-runs

• Solargraph no longer displays false warnings about spec_helper files in tests.
• Solargraph does not check whether required Ruby files exist. This is still checked through the test suite.

Action items

• (If applicable) Add documentation to the handbook pages for Triage Operations =>
• (If applicable) Identify the affected groups and how to communicate to them:
  • /cc @person_or_group =>
  • Relevant Slack channels =>
  • Engineering week-in-review