[Improvement] Merge runtags, version and customfilters into customfilters

Nobody has claimed this yet.

Currently there are 3 types of "filters" you can provide to your runs.
1. `tags` which are set on the `output.xml` level and with the `--outputfolder` or `--output` parameter.
2. `projectversion` which you set with `--projectversion` also on the output level but with a custom parameter.
3. `customfilters` which you set with `--customfilters` and these are also on the output level but again with another custom parameter.

The "nicest" implementation is the customfilters one since that one scales infinitely and can also handle filters like `tags` or `versions` that are now separately added. In the future it would be nice to make this all into a single CLI option which is `customfilters` so that everything works the same.

Design decisions when implementing:
- How to handle backward compatibility? Is that even possible at all?
- How to handle the overview page filters (version and others?), suggestion by @HuntTheSun in this comment could be good: https://github.com/MarketSquare/robotframework-dashboard/pull/286#issuecomment-4438078921
- How to handle the server implementation since everything is separated there now

Start by tracing the existing handling of --outputfolder, --output, --projectversion, and --customfilters, including how filters are represented in output.xml. Then inspect the overview page and server implementation, and read the linked pull request comment for the proposed filter behavior. Done means agreeing on backward compatibility and unifying the CLI, overview filters, and server handling.

Written by the indexing model from the issue text.