releases: Mark 2.13 as a new LTS release.

releases: Mark 2.13 as a new LTS release.

2.5 release is 4.5 years old and I'm not aware of anyone who actually
uses it today.  Release process documentation says that there is no
strict time period for nominating a new LTS release and that usually
it happens once in a two years.  So, proposing to nominate 2.13 as
our new LTS release since it's a first release that doesn't include
OVN inside, so we will formally not have to support it in this
repository in case there are major issues that might be hard to fix.

Suggested-by: Ben Pfaff <blp@ovn.org>
Acked-by: Flavio Leitner <fbl@sysclose.org>
Acked-by: Ian Stokes <ian.stokes@intel.com>
Acked-by: Kevin Traynor <ktraynor@redhat.com>
Reviewed-by: Simon Horman <simon.horman@netronome.com>
Signed-off-by: Ilya Maximets <i.maximets@ovn.org>