Rename NEVER_SUGGEST annotation to LAST_RESORT_SUGGESTION

With change Id71314cbd the behavior of the NEVER_SUGGEST annotation was
changed, so that code owners the are annotated with NEVER_SUGGEST are
suggested if otherwise the suggestion result would be empty. This is a
bit confusing since this behavior doesn't match the name of the
NEVER_SUGGEST annotation which implies that code owners with this
annotation would never be suggested. To remove this confusion the
NEVER_SUGGEST annotation is renamed to LAST_RESORT_SUGGESTION which
describes the behavior much better. Code owners with this annotation are
suggested as a last resort if no other code owners are available.

Renaming the annotation is a backwards incompatible change and code
owner config files that already use the NEVER_SUGGEST annotation need to
be adapted manually. This is fine, since as far as we know this is only
used at a single place so far.

Signed-off-by: Edwin Kempin <ekempin@google.com>
Change-Id: I74726784ee7138f40aeb9d783e5ee52c917f2ce7