docs: removed outdated migration documentation

[phalgunv]

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• The commit message follows our guidelines: https://github.com/angular/angular/blob/main/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
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• Bugfix
• Feature
• Code style update (formatting, local variables)
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Other information

closes #46236

[dgp1130]

I don't have much context on this, but the doc seems pretty old. @alan-agius4 is this still relevant or should we drop it?

[alan-agius4]

LGTM.

[alan-agius4]

@dgp1130 we can definitely drop this.

[dgp1130]

Thanks for the confirmation @alan-agius4, and for taking the time to clean up the obsolete doc @phalgunv!

[phalgunv]

Thanks @dgp1130 and @alan-agius4 for approving my first PR! What would be the next steps?

[dgp1130]

@phalgunv, we have a "caretaker" rotation that merges PRs which area ready. The action: merge label marks this as ready to merge, so the caretaker should merge it today or tomorrow. Nothing you need to do. :D

[alxhub]

This PR was merged into the repository by commit 385b5ec.

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