roachtest/mixedversion: add support for required upgrades

Currently, the framework supports specifying a _minimum_ number of upgrades (`mixedversion.MinUpgrades`). However, there is no mechanism to specify _required_ upgrades. For example, a test may need to verify that a specific upgrade path doesn't regress [1]. This could be expressed via the `mixedversion.MustUpgrades("22.1", "23.2")`. (The reason we can't express the same fact using `MinUpgrades` is because its interpretation is relative to the _current_ version.)

[1] https://github.com/cockroachdb/cockroach/pull/136074

Jira issue: CRDB-44931

Start in the roachtest/mixedversion framework by reading the existing mixedversion.MinUpgrades handling and the linked upgrade-path example. Add support for expressing a required path such as mixedversion.MustUpgrades("22.1", "23.2"); done means a test can require that path independently of the current version and existing mixed-version behavior remains intact.

Written by the indexing model from the issue text.