RPC: Don't send base64 TXs to old clusters (bp #13047)

[mergify]

This is an automated backport of pull request #13047 done by Mergify

Cherry-pick of 86a9821 has failed:

On branch mergify/bp/v1.3/pr-13047
Your branch is up to date with 'origin/v1.3'.

You are currently cherry-picking commit 86a982150.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:

	modified:   client/src/mock_sender.rs
	modified:   client/src/rpc_client.rs

Unmerged paths:
  (use "git add <file>..." to mark resolution)

	both modified:   Cargo.lock
	both modified:   client/Cargo.toml

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[CriesofCarrots]

@t-nelson , I think you might have pushed to the wrong branch here!

[mergify]

automerge label removed due to a CI failure

[codecov]

Codecov Report

Merging #13062 into v1.3 will increase coverage by 0.0%.
The diff coverage is 75.7%.

@@          Coverage Diff           @@
##            v1.3   #13062   +/-   ##
======================================
  Coverage   82.0%    82.0%           
======================================
  Files        362      362           
  Lines      84475    84507   +32     
======================================
+ Hits       69337    69372   +35     
+ Misses     15138    15135    -3