Explain that RHEL 6 is not a supported OS due to the lack of appropriate GLIBC libraries

[asachs01]

Context

During a call, we discovered that a user was running Graylog on a RHEL 6 instance. With @dennisoelkers's help, we found that RHEL 6 lacks the libraries needed for Graylog to run reports correctly.

Expected Content

We should ensure that the docs note that RHEL 6 isn't supported (it's EOL'd anyways)

Your Environment

• Graylog Version: 3.2.4
• Elasticsearch Version: 5.X
• MongoDB Version: 4.X
• Operating System: RHEL 6
• Browser version: Firefox 86