v21.0.0 - Angular v21 updates & security updates. (#18)

* Version uptick

* `ng update @angular/cli`

* `ng update @angular/material`

* `ng update angular-eslint`

* Update README

* Update peer dep range.

Author: CodyTolene

README.md

@@ -354,6 +354,7 @@ Line numbers are enabled by default, but you can turn them off like so.
 
 | Angular version | @proangular/ngx-gist | Install                              |
 | --------------- | -------------------- | ------------------------------------ |
+| v21             | v21.x                | `ng add @proangular/ngx-gist@^21`    |
 | v20             | v20.x                | `ng add @proangular/ngx-gist@^20`    |
 | v19             | v19.x                | `ng add @proangular/ngx-gist@^19`    |
 | v18             | ------               | Untested                             |