docs: release notes for the v21.2.1 release

[alan-agius4]

Cherry-picks the changelog from the "21.2.x" branch to the next branch (main).

[gemini-code-assist]

Code Review

This pull request adds the release notes for version 21.2.1 to the changelog. I've provided a few suggestions to improve formatting consistency in the change descriptions. Specifically, I've recommended wrapping package names, options, and property names in backticks to align with the existing formatting for commands and function calls within the same file.

_{Note: Security Review has been skipped due to the limited scope of the PR.}

[gemini-code-assist]

For consistency and better readability, it's good practice to wrap tool names like npm in backticks, similar to how other commands and tools are formatted in this file.

Suggested change

	| [465073bc1](http://github.com/angular/angular-cli/commit/465073bc1b2b0e9fa594698651a9e0afe747a74a) | fix | disable npm update notifier in package manager host |
	| [465073bc1](http://github.com/angular/angular-cli/commit/465073bc1b2b0e9fa594698651a9e0afe747a74a) | fix | disable `npm` update notifier in package manager host |

[gemini-code-assist]

For consistency with other entries in the changelog (e.g., ng add, vi.mock), it's good practice to wrap option names in backticks for better readability. The minimal option should be enclosed in backticks.

Suggested change

	| [72d466aa0](http://github.com/angular/angular-cli/commit/72d466aa04d4d0cc4d654410bcb6dd44f0de3357) | fix | prevent adding test dependencies when minimal option is enabled |
	| [72d466aa0](http://github.com/angular/angular-cli/commit/72d466aa04d4d0cc4d654410bcb6dd44f0de3357) | fix | prevent adding test dependencies when `minimal` option is enabled |

[gemini-code-assist]

For consistency and better readability, package names should be wrapped in backticks, similar to how commands and other code identifiers are formatted in this changelog.

Suggested change

	| [0019d1c8e](http://github.com/angular/angular-cli/commit/0019d1c8e1494295a754063dbf936e1cd40d05bd) | fix | update copy-webpack-plugin to v14.0.0 |
	| [0019d1c8e](http://github.com/angular/angular-cli/commit/0019d1c8e1494295a754063dbf936e1cd40d05bd) | fix | update `copy-webpack-plugin` to v14.0.0 |

[gemini-code-assist]

To maintain consistency in formatting within the changelog, property names like browsers should be enclosed in backticks for improved readability.

Suggested change

	| [584f6a2d9](http://github.com/angular/angular-cli/commit/584f6a2d95ac4bdd9f20d918c6700ea79227cc92) | fix | treat empty browsers array as undefined in unit-test builder |
	| [584f6a2d9](http://github.com/angular/angular-cli/commit/584f6a2d95ac4bdd9f20d918c6700ea79227cc92) | fix | treat empty `browsers` array as undefined in unit-test builder |