refactor: adapting transfer metadata bytes field to memo string (backport #2595)

[mergify]

This is an automatic backport of pull request #2595 done by Mergify.
Cherry-pick of 05685b3 has failed:

On branch mergify/bp/release/v6.0.x/pr-2595
Your branch is up to date with 'origin/release/v6.0.x'.

You are currently cherry-picking commit 05685b3.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   CHANGELOG.md
	modified:   docs/ibc/proto-docs.md
	modified:   modules/apps/transfer/client/cli/tx.go
	modified:   modules/apps/transfer/ibc_module.go
	modified:   modules/apps/transfer/keeper/mbt_relay_test.go
	modified:   modules/apps/transfer/keeper/msg_server.go
	modified:   modules/apps/transfer/keeper/msg_server_test.go
	modified:   modules/apps/transfer/keeper/relay.go
	modified:   modules/apps/transfer/transfer_test.go
	modified:   modules/apps/transfer/types/events.go
	modified:   modules/apps/transfer/types/msgs.go
	modified:   modules/apps/transfer/types/msgs_test.go
	modified:   modules/apps/transfer/types/packet.go
	modified:   modules/apps/transfer/types/packet.pb.go
	modified:   modules/apps/transfer/types/packet_test.go
	modified:   modules/apps/transfer/types/tx.pb.go
	modified:   proto/ibc/applications/transfer/v1/tx.proto
	modified:   proto/ibc/applications/transfer/v2/packet.proto

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   modules/apps/29-fee/transfer_test.go
	both modified:   modules/apps/transfer/keeper/relay_test.go

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/github/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

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