replace deprecated "service" and "yum" commands within the linux install sections

On this page, [https://docs.influxdata.com/influxdb/v2.0/install/?t=Linux]( https://docs.influxdata.com/influxdb/v2.0/install/?t=Linux) under the section "**_Install InfluxDB as a service with systemd_**" there are several commands using the "`service`" command which has been deprecated many years ago. It may be better to use proper systemd commands in their place.

For example...

The document refers to starting a service using "`sudo service influxdb start`" though that command is currently redirected via systemd to the proper command, I believe it would be better to actually list the proper command. "`sudo systemctl start influxdb`" or better yet, "`sudo systemctl start influxdb --now`" which will start and enable it so it restarts at reboot. (in one command) Another deprecated command, but not as outdated as the first, is the use of "`yum`" instead of "`dnf`" for centos, rhel, fedora.. even though in modern RHEL based operating systems have linked yum to the dnf command, that could be removed at any point in time so dnf is recommended here.

Another very large inconsistency within the documentation for influxdb on linux is that the instructions on the same page listed above suggest using wget to download the rpm or db package and there is a link to the downloads page. If you select RHEL or CentOS on he downloads page you get instructions on how to create an influxdb repository and pull the packages from there. This is a far better strategy which also aids folks during the update/upgrade process. The instructions for linux (link above) should have the wget instructions removed and the user should be directed to the download page which will contain the proper downloading method and commands for their particular operating system.

Start with the Linux section of the InfluxDB v2.0 install page and compare its service, package, and download instructions with the linked downloads page. Update the documented commands and installation path so they are consistent with the current OS-specific guidance, then verify the rendered Linux instructions are complete and internally consistent.

Written by the indexing model from the issue text.