Switch RedHat-likes to FdoDetect, add some RHEL clones

[cottsay]

Switch all remaining RedHat distributions to use FdoDetect instead of the custom /etc/redhat-release detection.

Also add support for AlmaLinux, Amazon Linux, Oracle Linux, and Rocky Linux.

Results of running python3 -m rospkg.os_detect on each of these platforms:

• AlmaLinux 8:

  OS Name:     almalinux
  OS Version:  8.3
  OS Codename: purple manul
  

• Amazon Linux 2:

  OS Name:     amazon
  OS Version:  2
  OS Codename: 2
  

• CentOS 7:

  OS Name:     centos
  OS Version:  7
  OS Codename: core
  

• CentOS 8:

  OS Name:     centos
  OS Version:  8
  OS Codename: 8
  

• EulerOS:

  OS Name:     euleros
  OS Version:  2.0
  OS Codename: sp3
  

• Oracle Linux 8.3:

  OS Name:     oracle
  OS Version:  8.3
  OS Codename: 8.3
  

• RHEL 7:

  OS Name:     rhel
  OS Version:  7.6
  OS Codename: maipo
  

• RHEL 8:

  OS Name:     rhel
  OS Version:  8.2
  OS Codename: ootpa
  

• Rocky Linux

  OS Name:     rocky
  OS Version:  8
  OS Codename: 8
  

[tfoote]

Moving forward on this makes a lot of sense. It's a lot cleaner and simpler.

RHEL 6 is over 10 years old and is into ELS so not officially supported. That's fine to drop it for us. If you're using the ELS you can choose to pin an older version for your older platform.