Improve `VersionedMigration` naming conventions (#2264)

As suggested by @ggwpez
(https://github.com/paritytech/polkadot-sdk/pull/2142#discussion_r1388145872),
remove the `VersionChecked` prefix from version checked migrations (but
leave `VersionUnchecked` prefixes)

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Co-authored-by: command-bot <>