Changelog category (leave one):

• New Feature

Changelog entry (a user-readable short description of the changes that goes to CHANGELOG.md):

Support alter from String to JSON. This PR also changes the serialization of JSON and Dynamic types to new version V2. Old version V1 can be still used by enabling setting merge_tree_use_v1_object_and_dynamic_serialization (can be used during upgrade to be able to rollback the version without issues).

Documentation entry for user-facing changes

• Documentation is written (mandatory for new features)

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