Update angular templates with latest angular 21.0 and latest igniteui-angular 21.0

[Lipata]

Update angular & igniteui-angular packages to 21.0.0

• currently in alpha: https://github.com/IgniteUI/igniteui-angular/releases/tag/21.0.0-alpha.1)

Based on the analysis of PR #1382, which updated from 19.0 to 19.1, make similar changes for version 21.0.

• Analyze repository structure and current state
• Identify files that need version updates based on PR chore: update ignite and ng to 19.1; update other packages; #1382 pattern
• Update constants.ts with new igniteui-angular version (21.0.0)
• Update Angular packages in igx-ts template package.json (to ~21.0.0)
• Update Angular packages in igx-ts-legacy template package.json (to ~21.0.0)
• Update Angular CLI and related dev dependencies to compatible versions
• Verify build and lint still work
• Restore yarn.lock to original state to remove any lock file changes