Instructions out of date for RHEL 8/9

[mcandre]

When I try to install Ruby on RHEL (8/9) with ASDF, then yum and dnf note that there is no longer a package called libyaml-devel.

In recent versions of RHEL, the package is hidden by default. RHEL users will need to explicitly enable some package groups:

# RHEL 8 based distros:
dnf config-manager --set-enabled powertools

# RHEL 9 based distros:
dnf config-manager --set-enabled crb

dnf install libyaml-devel

Per rvm/rvm#5288 (comment)

For comparison, RVM uses Ruby's built-in libyaml implementation:

rvm/rvm#2998 (comment)

Also, the GCC version is out of date.

This issue appears to be specific to the underlying rbenv/ruby-build system:

rbenv/ruby-build#2437