feat(migrations): add migration for circular theme to rename base circle color property

[desig9stein]

Closes #17197

Additional information (check all that apply):

• Bug fix
• New functionality
• Documentation
• Demos
• CI/CD

Checklist:

• All relevant tags have been applied to this PR
• This PR includes unit tests covering all the new code (test guidelines)
• This PR includes API docs for newly added methods/properties (api docs guidelines)
• This PR includes feature/README.MD updates for the feature docs
• This PR includes general feature table updates in the root README.MD
• This PR includes CHANGELOG.MD updates for newly added functionality
• This PR contains breaking changes
• This PR includes ng update migrations for the breaking changes (migrations guidelines)
• This PR includes behavioral changes and the feature specification has been updated with them

[Copilot]

Pull request overview

Adds the missing 21.2.0 migration entrypoint and theme-change config so ng update can rename the Circular theme’s circular progress “base circle color” parameter to the new name, addressing the reported ng update errors for 21.2.0.

Changes:

• Introduces the update-21_2_0 migration entrypoint that runs UpdateChanges for v21.2.0.
• Adds a theme change mapping to rename $base-circle-color to $track-color for circular-theme.
• Adds a schematic test validating the SCSS rename behavior.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
projects/igniteui-angular/migrations/update-21_2_0/index.ts	New 21.2.0 migration entrypoint invoking UpdateChanges.
projects/igniteui-angular/migrations/update-21_2_0/index.spec.ts	Adds unit test coverage for the SCSS parameter rename migration.
projects/igniteui-angular/migrations/update-21_2_0/changes/theme-changes.json	Declares the $base-circle-color → $track-color change for circular-theme.

[Copilot]

testFilePath is declared with single quotes, so the ${theme} placeholder is never interpolated. This makes the test create/read a literal file named ${theme}.component.scss rather than a theme-specific path, which is misleading and won’t scale if more themes are added. Build the path with a template literal inside the loop (or use a helper function) so it actually includes the current theme value.