cdc: mixed version roachtest never creates changefeed in a mixed-version state

The `cdc/mixed-versions` roachtest registers `createChangeFeed` as an `OnStartup` hook, meaning the changefeed is created on the initial homogeneous cluster before any nodes are upgraded. The test validates that an existing changefeed survives the upgrade, but never tests *creating* a changefeed when nodes are running different binary versions.

The changefeed creation should also be tested `InMixedVersion` to ensure that the creation code path works when the cluster is in a mixed binary state.

**Relevant code:**
- [`OnStartup("start changefeed", ...)`](https://github.com/cockroachdb/cockroach/blob/4442354d04be76f56cbbdb06f76894fb3dcf4fbf/pkg/cmd/roachtest/tests/mixed_version_cdc.go#L599)
- [`createChangeFeed` implementation](https://github.com/cockroachdb/cockroach/blob/4442354d04be76f56cbbdb06f76894fb3dcf4fbf/pkg/cmd/roachtest/tests/mixed_version_cdc.go#L387-L434)

**Motivation:** This is the same gap identified for LDR in #167156.

Epic: none

Jira issue: CRDB-63393