release-23.2: roachtest: port follower-reads/mixed-version/single-region to new framework

[nvanbenschoten]

Backport 1/1 commits from #115317.

/cc @cockroachdb/release

Release justification: testing only

---

Fixes #115133.

This commit ports the follower-reads/mixed-version/single-region roachtest to the new mixed-version framework, as described in #110528.

One note for reviewers is that the new mixed-version framework requires clusters to have 4 nodes (unless we want to disable fixtures), so part of the work here was to adapt the test to work with 4 nodes. That's worthwhile anyway, because testing the case where a gateway has no replica (leaseholder or follower) for the table is interesting.

Release note: None

[blathers-crl]

Thanks for opening a backport.

Please check the backport criteria before merging:

• Backports should only be created for serious
  issues or test-only changes.
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  here).
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If your backport adds new functionality, please ensure that the following additional criteria are satisfied:

• There is a high priority need for the functionality that cannot wait until the next release and is difficult to address in another way.
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[cockroach-teamcity]

This change is