ceph.spec.in: call %enable_devtoolset11 on centos8

ceph.spec.in: call %enable_devtoolset11 on centos8

before this change %enable_devtoolset11 is called only when building
with crimson on centos8.

after this change %enable_devtoolset11 is called when building on
centos8. because we've started using gcc-toolset-11 for building
rpm packages on centos8 after the C++20 migration. so, to build
with gcc-11, we need to enable it.

also, because gcc-toolset-11 is used, we have to disable
annotated_build.

Signed-off-by: Kefu Chai <tchaikov@gmail.com>