Fixes AngularJS mentions as migration is complete

[rohanmalhotracodes]

I’ve made the following AngularJS cleanup updates related to this issue:

• Learning-Resources.md
  Replaced the old “AngularJS/Angular” wording with “Angular”, and updated the learning resources so they point only to Angular material.

• Frontend-tests.md
  Removed AngularJS-specific references and examples (including AngularJS-only terminology and testing guidance), and updated the document so it consistently uses Angular terminology and Angular testing patterns.

• Overview-of-the-Oppia-codebase.md
  Updated the frontend overview to describe the frontend as Angular-based, removed AngularJS-specific comparison language, and simplified the MVC mapping so it uses Angular terminology only.

I also wanted to mention that I think Angular-Migration.md may no longer be necessary and could potentially be removed since the migration is now considered complete.

Fix #511

[rohanmalhotracodes]

@HardikGoyal2003 @jayam04 gentle reminder, thanks and apologies for taking your time.

[HardikGoyal2003]

@rohanmalhotracodes Please remove the Angular-migration page also.

[HardikGoyal2003]

I think this file also needs to be updated: https://github.com/oppia/oppia/wiki/Coding-style-guide

[HardikGoyal2003]

Also try looking into this: https://github.com/search?q=repo%3Aoppia%2Foppia+mainpage&type=wikis

[rohanmalhotracodes]

Addressed the issues stated by sir
@HardikGoyal2003 PTAL

[HardikGoyal2003]

@rohanmalhotracodes We need to keep this PR on the stand by for some time, as we are in the process of removing some remaining architecture of AngularJS, where these docs will be needed to refer. So for now, let this PR be open, once we are ready to remove it, I will ping you. Thanks!

[rohanmalhotracodes]

Understood. Thanks.