Section about deprecated onDemand and onAbort callback arguments (#2330)

Section about deprecated onDemand and onAbort callback arguments (#2330)

* Added deprecated onDemand and onAbort callback arguments to migration guide

The  onDemand and onAbort  callbacks for router.push and .replace were deprecated in v4. I added a section to the migration guide explaining that users should replace them with the thenable Promise the methods return.

* Apply suggestions from code review

* Update packages/docs/guide/migration/index.md [skip ci]

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Co-authored-by: Eduardo San Martin Morote <posva@users.noreply.github.com>