Remove serdes support for legacy metrics (#43272)

docs/api-documentation.md

docs/api/legacy-metric.md

docs/developers-guide/api-changelog.md

...ise/backend/src/metabase_enterprise/serialization/cmd.clj

...se/backend/src/metabase_enterprise/serialization/dump.clj

...se/backend/src/metabase_enterprise/serialization/load.clj

...e/backend/src/metabase_enterprise/serialization/names.clj

...ckend/src/metabase_enterprise/serialization/serialize.clj

.../backend/src/metabase_enterprise/serialization/upsert.clj

...ckend/src/metabase_enterprise/serialization/v2/models.clj

...ackend/test/metabase_enterprise/models/entity_id_test.clj

...kend/test/metabase_enterprise/serialization/load_test.clj

...end/test/metabase_enterprise/serialization/names_test.clj

...test/metabase_enterprise/serialization/serialize_test.clj

...kend/test/metabase_enterprise/serialization/test_util.clj

...nd/test/metabase_enterprise/serialization/upsert_test.clj

...nd/test/metabase_enterprise/serialization/v2/e2e_test.clj

...est/metabase_enterprise/serialization/v2/extract_test.clj

...d/test/metabase_enterprise/serialization/v2/load_test.clj

src/metabase/cmd/copy.clj