docs: update links to the Angular update guide

[MeAkib]

The update guide moved from https://update.angular.dev/ to https://angular.dev/update-guide. Update the README, the ng update command description and error messages, and the @angular/build version-mismatch error to point to the new location. The v query parameter is still supported by the new page.

[gemini-code-assist]

Code Review

This pull request updates the Angular upgrade guide URL from https://update.angular.dev/ to https://angular.dev/update-guide across multiple files, including the README, CLI command descriptions, error messages, and template files. I have no feedback to provide as the changes are straightforward and correct.

[alan-agius4]

Using https://update.angular.dev/ is intentional. Having a dedicated subdomain acts as a stable redirect layer that decouples published CLI versions from the internal URL structure of angular.dev. Since CLI packages remain installed in projects for a long time (especially when running ng update from older versions), pointing to update.angular.dev allows the documentation site to reorganize or move the update guide path in the future without breaking links baked into older CLI releases, while also keeping URLs in terminal output shorter and easier to read.