ipsec: promote library as stable

ipsec: promote library as stable

Since librte_ipsec was first introduced in 19.02 and there were no changes
in it's public API since 19.11, it should be considered mature enough to
remove the 'experimental' tag from it.
The RTE_SATP_LOG2_NUM enum is also being dropped from rte_ipsec_sa.h 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>
Acked-by: Ray Kinsella <mdr@ashroe.eu>
Acked-by: Akhil Goyal <akhil.goyal@nxp.com>