ceph.spec.in: drop RHEL <= 8 support

ceph.spec.in: drop RHEL <= 8 support

RHEL 8 and CentOS 8 left the supported test matrix in 2023 (449db416,
7a1dce1e). RHEL 8 has also been unbuildable since CMakeLists.txt began
requiring Python >= 3.9: it ships python 3.6 by default, so cmake refuses
to configure. The spec still carried the el8 conditionals, so assume
rhel >= 9 and drop them:

- remove branches reachable only on rhel <= 8: the gperftools 2.6.1
  BuildRequires, the tracker-36508 tcmalloc-libs requirement, and the
  dataclasses backport, which also targeted that dead python 3.6
- collapse "rhel >= 8" and "rhel >= 9" to plain "rhel"; with the floor at
  9 this is a no-op for rhel 9/10, fedora, suse and openEuler
- keep the rhel 9-vs-10 checks (== 9, < 10, >= 10)

Signed-off-by: Kefu Chai <k.chai@proxmox.com>