Allow filtering runners by version prefix on admin runners page

Admins sometimes want to filter runners by version prefix, i.e. give me all runners from version 15.x.

We could introduce a new filter option called Version starts with:

The search would be performed on ci_runner_machines.version, not ci_runners.version. To better support searches on runner versions, we should replace the index_ci_runner_machines_on_version index with a trigram version index_ci_runner_machines_on_version_trigram.