install instructions don't mention repository setup

installation instructions for older influxdb versions, like https://docs.influxdata.com/influxdb/v1.8/introduction/install/, mention how people can setup a debian/RHEL repository. The docs for version 2.4 don't mention that. Instead they suggest to download the rpm/deb and install it by hand. I think that's bad practice because people cannot easily upgrade in the future/their package manager won't inform them about available updates.

However, I noticed that the repo setup is documented at https://portal.influxdata.com/downloads/ . maybe that should be mentioned at https://docs.influxdata.com/influxdb/v2.4/install/ (and recommended for people using packages?)

also the docs seem to switch between the influxdb.key and influxdb2.key. Can you maybe explain the difference?
* https://docs.influxdata.com/influxdb/v1.8/introduction/install/ - uses influxdb.key for influxdb1.8
* https://docs.influxdata.com/influxdb/v2.0/install/ - used influxdb2.key for influxdb2
* https://portal.influxdata.com/downloads/ - uses influxdb.key for influxdb2.4

##### Relevant URLs
- https://docs.influxdata.com/influxdb/v2.4/install/
- _Provide other relevant URLs (project issues, community threads, etc.)_

Start with the InfluxDB v2.4 installation page and compare it with the v1.8, v2.0, and InfluxData downloads pages linked in the issue. Clarify how repository-based installation should be presented for Debian/RHEL users and explain the influxdb.key versus influxdb2.key naming. Done means the v2.4 instructions point users to the appropriate repository guidance with consistent key information.

Written by the indexing model from the issue text.