bpf: promote library as stable

bpf: promote library as stable

The BPF lib was introduced in 18.05.
There were no changes in its public API since 19.11.
It should be mature enough to remove its 'experimental' tag.
RTE_BPF_XTYPE_NUM is also being dropped from rte_bpf_xtype to
avoid possible ABI problems in the future.

Signed-off-by: Conor Walsh <conor.walsh@intel.com>
Acked-by: Konstantin Ananyev <konstantin.ananyev@intel.com>