refactor(@angular-devkit/schematics): remove UpdateBuffer and rename UpdateBuffer2 to UpdateBuffer

[kyubisation]

This PR removes the internally built UpdateBuffer and renames UpdateBuffer2 (based on magic-string) to UpdateBuffer. This should have little to no impact for consumers.

BREAKING CHANGE:
The depracated UpdateBuffer has been removed and UpdateBuffer2 is renamed to UpdateBuffer. With this change the related and deprecated symbols ContentCannotBeRemovedException and Chunk have also been removed.

PR Checklist

Please check to confirm your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/angular/angular-cli/blob/main/CONTRIBUTING.md#-commit-message-guidelines
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Documentation content changes
• Other... Please describe:

What is the current behavior?

The deprecated UpdateBuffer instance is being used per default.

Issue Number: N/A

What is the new behavior?

The magic-string based UpdateBuffer2 is being used per default.

Does this PR introduce a breaking change?

• Yes
• No

Other information

See #21209 and #21110

[alan-agius4]

Thanks for this.

However, this needs some more work if you are up to it.

• Delete the legacy UpdateBuffer
• Rename UpdateBuffer2 to UpdateBuffer
• Add a breaking change note in the commit message?

[kyubisation]

I was unsure whether to directly delete the UpdateBuffer. If you think it's ok to remove, then I will do that and update it with a breaking change description. I will also remove the NG_UPDATE_BUFFER_LEGACY flag, as it would have no effect this way.

[kyubisation]

Should I also merge UpdateBufferBase with the renamed UpdateBuffer?

[alan-agius4]

Should I also merge UpdateBufferBase with the renamed UpdateBuffer?

No, as UpdateBufferBase is part of the public API while UpdateBuffer is private.

[kyubisation]

LinkedList is no longer used, but was not marked as deprecated, so I left it in (but added deprecation note).
Is the PR fine like this or should I change something else?

[alan-agius4]

Thanks for your contribution. One comment about the LinkedList otherwise LGTM

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