distro/rhel86: add CentOS Stream 8 as alias to RHEL 8.6

Removed old alias from RHEL 8.4.

To make aliasing distributions simpler, all distro-specific strings were
added to the local distribution type.

These were previously global constants of the distribution package and
alias alternatives were used conditionally. Now the two distributions
are predefined in global map.

CentOS exclusions:
- s390x arch
- ec2 image types: rhui client is not available

Signed-off-by: Achilleas Koutsou <achilleas@koutsou.net>

internal/distroregistry/distroregistry.go

@@ -23,9 +23,9 @@ var supportedDistros = []supportedDistro{
 	{fedora33.NewF36, fedora33.NewHostDistro},
 	{rhel8.New, rhel8.NewHostDistro},
 	{rhel84.New, rhel84.NewHostDistro},
-	{rhel84.NewCentos, rhel84.NewCentosHostDistro},
 	{rhel85.New, rhel85.NewHostDistro},
 	{rhel86.New, rhel86.NewHostDistro},
+	{rhel86.NewCentos, rhel86.NewCentosHostDistro},
 	{rhel90beta.New, rhel90beta.NewHostDistro},
 	{rhel90beta.NewRHEL90, rhel90beta.NewHostDistro},
 }