Fix GitHub project import filter box

What does this MR do?

Addresses: #36556 (closed)

Prior to this change, attempting to filter the list of projects for import from GitHub would clear the list, regardless of the filter value. The filtering logic would also perform a case-sensitive search for matches.

This MR:

1. Ensures the list of repos is refreshed after the filter is set
2. Changes the search to ignore case
3. Cleans up some unused methods in files being updated

Screenshots (strongly suggested)

Before	After
Before	After

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• 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