[release-18.0] CI: Lower resources used for TestVtctldMigrateSharded (#16875)

[vitess-bot]

Description

This is a backport of #16875

[vitess-bot]

Hello @mattlord, there are conflicts in this backport.

Please address them in order to merge this Pull Request. You can execute the snippet below to reset your branch and resolve the conflict manually.

Make sure you replace origin by the name of the vitessio/vitess remote

git fetch --all
gh pr checkout 16881 -R vitessio/vitess
git reset --hard origin/release-18.0
git cherry-pick -m 1 2e47aba034e965cb7fddf0f70fd440105650f704

[vitess-bot]

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[mattlord]

It turns out that this branch does NOT have the TestVtctldMigrateSharded test so I'm going to close this as it's not needed.

This has a dependency on #16813, so if that is eventually merged then this should be re-opened, rebased, and merged.