fix(zmr): install the previous2021 Source SDK 2013 build (#5147)

The public branch of Source SDK Base 2013 Dedicated Server (244310) is
now the 2025 SDK update, which Zombie Master: Reborn b6.1 does not run
on: the server falls back to LAN mode and never answers queries.

Use the previous2021 branch (build 6982827), the engine ZMR was built
for.

lgsm/config-default/config-lgsm/zmrserver/_default.cfg

@@ -150,7 +150,7 @@ sleeptime="0.5"
 appid="244310" # Source 2013 SDK
 steamcmdforcewindows="no"
 # SteamCMD Branch | https://docs.linuxgsm.com/steamcmd/branch
-branch=""
+branch="previous2021"
 betapassword=""
 # Master Server | https://docs.linuxgsm.com/steamcmd/steam-master-server
 steammaster="true"