roachtest: enable separate-process deployment in remaining mixedversion tests

`separate-process` deployments were added to the mixedversion framework in #130194. At that point, only a subset of tests were opted-in to this new deployment mode. Most tests that are known to fail (for non-obvious reasons) in that deployment mode had it disabled.

We should look at the failures more closely and enable separate-process on the following tests:

- [ ] `follower-reads/mixed-version/*`
- [x] `rebalance-load/*/mixed-version`
- [x] `backup-restore/mixed-version`
- [x] `import/mixed-versions`
- [x] `decommission/mixed-versions`
- [x] `validate-system-schema-after-version-upgrade` (only in local mode)
- [x] `acceptance/version-upgrade` (only in local mode)

Jira issue: CRDB-42310