kafka/server: Pass a string view to to consensus::step_down

During some recent refactoring in group.cc, we introduced a temporary sstring
in a chain of string_view params, introducing a UAF in some continuation-style
code in the raft layer. Restore the string_view to avoid this.

Note that AddressSanitizer caught the read on the transactional offset-commit
path, where a replication failure while still leader reaches this line.

Signed-off-by: Oren Leiman <oren.leiman@redpanda.com>