Remove CentOS 6 support

[jarretlavallee]

Use Case

CentOS 6 is EOL and no longer receiving maintenance updates. Testing this is causing CI failures and support should be removed.

Describe the Solution You Would Like

Remove CentOS from CI testing and the metadata.json

[jarretlavallee]

We fixed the CI issues in litmus with a workaround, but support is being removed soon. More info is in puppetlabs/litmusimage#26

The puppet-grafana and puppet-telegraf modules just came out with major releases that removed EL 6 and Debian 8.

https://forge.puppet.com/modules/puppet/grafana/changelog#v800-2021-02-20
https://forge.puppet.com/modules/puppet/telegraf/changelog#v400-2021-02-17

@suckatrash @genebean @Sharpie What are your thoughts on EOL OS support?

[Sharpie]

PE 2018.1 was the last supported PE release that allowed for RedHat 6 masters. Now that 2018.1 is EOL, we could cut a new major version that moves the minimum supported PE version to 2019.8 and drops RedHat 6.