Update Angular to v21

[NetWin]

Not much to say tbh. Clients need to update to Angular 21 aswell in order to use the latest version that's it.

[github-actions]

This repository uses semantic-release to automate the process of releasing new versions. The following changelog entries would be generated for the next release. Please review them to ensure they correctly reflect the changes in this pull request.
If you want to change the changelog entries, please edit the commit messages in this pull request following Conventional Commits guidelines. Use git rebase -i master to edit the commit history.

Changelog preview (the formatting is slightly different from the actual release):

# 21.0.0 (https://github.com/netwin/date-time-picker/compare/20.0.1...21.0.0) (2026-01-07)

### Features

    * update Angular to v21 ([05e5122](https://github.com/netwin/date-time-picker/commit/05e512227d10c956e798c78698fb67240f1c963a))

### BREAKING CHANGES

    * Clients need to update to Angular 21 aswell in order to use the latest version

[github-actions]

🎉 This PR is included in version 21.0.0-rc.1 🎉

The release is available on:

• npm package (@next dist-tag)
• GitHub release

Your semantic-release bot 📦🚀

[github-actions]

🎉 This PR is included in version 21.0.0-rc.2 🎉

The release is available on:

• npm package (@next dist-tag)
• GitHub release

Your semantic-release bot 📦🚀

[github-actions]

🎉 This PR is included in version 21.0.0 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

Your semantic-release bot 📦🚀