Fix compilation warnings with libselinux 3.1 in contrib/sepgsql/

Fix compilation warnings with libselinux 3.1 in contrib/sepgsql/

Upstream SELinux has recently marked security_context_t as officially
deprecated, causing warnings with -Wdeprecated-declarations.  This is
considered as legacy code for some time now by upstream as
security_context_t got removed from most of the code tree during the
development of 2.3 back in 2014.

This removes all the references to security_context_t in sepgsql/ to be
consistent with SELinux, fixing the warnings.  Note that this does not
impact the minimum version of libselinux supported.

This has been applied first as 1f32136 for 14~, but no other branches
got the call.  This is in line with the recent project policy to have no
warnings in branches where builds should still be supported (9.2~ as of
today).  Per discussion with Tom Lane and Álvaro Herrera.

Reviewed-by: Tom Lane
Discussion: http://postgr.es/m/20200813012735.GC11663@paquier.xyz
Discussion: http://postgr.es/m/20221103181028.raqta27jcuypor4l@alvherre.pgsql
Backpatch-through: 9.2