[restapi]: Don't use python/python2 for restapi start scripts (#10285)

Python 2 isn't installed by default in Buster and Bullseye containers,
and the scripts/modules can be used with Python 3, so make sure Python 3
is used.

Why I did it
After the Buster and Bullseye upgrade for the restapi container, processes will no longer start because supervisord is trying to call python and python2, both of which are unavailable.

Signed-off-by: Saikrishna Arcot <sarcot@microsoft.com>

dockers/docker-sonic-restapi/supervisord.conf

@@ -4,7 +4,7 @@ logfile_backups=2
 nodaemon=true
 
 [eventlistener:dependent-startup]
-command=python -m supervisord_dependent_startup
+command=python3 -m supervisord_dependent_startup
 autostart=true
 autorestart=unexpected
 startretries=0
@@ -13,7 +13,7 @@ events=PROCESS_STATE
 buffer_size=1024
 
 [eventlistener:supervisor-proc-exit-listener]
-command=python2 /usr/bin/supervisor-proc-exit-listener --container-name restapi
+command=/usr/bin/supervisor-proc-exit-listener --container-name restapi
 events=PROCESS_STATE_EXITED,PROCESS_STATE_RUNNING
 autostart=true
 autorestart=false