Upgrade: InfluxDB 2 paths options usability

InfluxDB 2 upgrade needs two options for specifying target paths. Both options are the same as `influxd run` uses:
- `-e | --engine-path`, path to the engine folder, default `/.influxdbv2/engine`
- `-b | --bolt-path`, path to the bolt DB file, default `/.influxdbv2/influxdb.bolt`

The most confusing thing seems to be the bolt DB path. A user needs to add a full path to the specific file, which doesn't exist yet. The improvement would be specifying just a folder, e.g., `--bolt-dir`.

However, it would be more clear if there is is just a single option e.g. `--v2-dir` to specify ve folder, default `/.influxdbv2/`. And implementation would just extend that into two necessary paths.

Start at the `influxd run` option definitions and the InfluxDB 2 upgrade entry point, then trace how `--engine-path` and `--bolt-path` are consumed. Clarify whether the accepted interface is a single `--v2-dir` or a directory-based bolt option; done means the chosen paths are documented, default correctly, and covered by upgrade tests.

Written by the indexing model from the issue text.