Update keyring migrate documentation for v0.42 and version specific migrations

[clevinson]

Summary

Keyring migrate commands were fixed & documented for the Hub upgrade path (0.37 -> 0.41) #8639, but as the keys migrate command is not explicit about versions, we should document (both in docs & CLI helpers) under which circumstances, and which specific versions we need to run keys migrate.

cc @AmauryM

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[aaronc]

Should this have a milestone tagged @clevinson ?

[clevinson]

@aaronc added it to v0.41.x (which may get renamed to v0.42.x soon...) as its important the docs here are live on stargate docs for non-hub chains that may be coming from other versions that 0.37.