Suite: Fix old solutions removing problem due to solution file extension mismatch #21227
BackDescription
Resolves #21185
The reason was that old solutions ended with .sln and the new solutions end with .slnx, so it was not remove the existing solutions. This fixes the problem.
Checklist
- I fully tested it as developer / designer and created unit / integration tests
- I documented it (or no need to document or I will create a separate documentation issue)
- I worked with the design team to get their idea for UI/UX design (or no need to design for this PR)
- I've assigned reviewer, related labels and set a milestone for this PR