[VUFIND-1752] Remove legacy (pre-10.x) upgrade support from release 11

[demiankatz]

This PR simplifies the UpgradeController and VuFind\Config\Upgrade class to remove support for upgrading pre-10.0 releases.

The work could be taken further; we need to decide just how far we want to take the cleanup. Some things to consider:

• Should we remove the "metadata upgrade" step from the upgrade tool? This should not be needed at present, but it might be useful to fix unanticipated problems in the future.
• Should we remove all references to pre-10.0 settings from the config upgrade class? I've removed logic that references specific versions, but there is still logic to address settings that were removed or changed before release 10.0. This could likely be simplified, but performing the simplification would require some time-consuming analysis.
• Should we remove support for non-in-place config upgrades? At this point, all upgrades will be in-place, so logic could be further simplified.

TODO:

• Decide whether more work should be done in the scope of this PR based on the questions above.
• Update changelog when merging.
• Resolve VUFIND-1752 when merging.