CI: Lower resources used for TestVtctldMigrateSharded

[mattlord]

Description

We added a new TestVtctldMigrateSharded test in #16769 and since then it's become our most flaky test in Launchable. This then causes the vreplication_migrate_vdiff2_convert_tz GitHub Actions workflow to fail regularly.

Example case where it failed two times in a row: https://github.com/vitessio/vitess/actions/runs/11130696603/attempts/1?pr=16860

The failures seem to be resource related as things stop progressing and we eventually time out:

2024-10-01T18:37:01.4154857Z I1001 18:03:39.125028   44394 vtctldclient_process.go:75] Executing vtctldclient with command: vtctldclient --server 127.0.0.1:25999 CreateKeyspace rating --sidecar-db-name __vt_e2e-test (attempt 1 of 10)
2024-10-01T18:37:01.4155050Z panic: test timed out after 30m0s
2024-10-01T18:37:01.4155253Z 	running tests:
2024-10-01T18:37:01.4155437Z 		TestVtctldMigrateUnsharded (30m0s)
...

This test creates two sharded Vitess clusters, so in this PR we hope to address this flakiness by lowering the resources required by eliminating replica and rdonly tablets from the two sharded Vitess clusters it creates.

We should backport this test-only change to v18 as the noted PR was backported to v18.

I ran the test 10 times in the PR and it passed every time: https://github.com/vitessio/vitess/actions/runs/11143601365/job/30969074871?pr=16875

Related Issue(s)

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
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• Documentation was added or is not required

[vitess-bot]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 69.41%. Comparing base (978c59d) to head (6dbd295).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16875      +/-   ##
==========================================
- Coverage   69.43%   69.41%   -0.02%     
==========================================
  Files        1571     1571              
  Lines      203195   203195              
==========================================
- Hits       141079   141054      -25     
- Misses      62116    62141      +25     

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