init: replace deprecated PermissionsStartOnly systemd directive

PermissionsStartOnly is deprecated [1]. This removes the directives
and instead we prefixes the value of the ExecStartPre directive with
'!', which means the executable, 'chgrp' in this case, is run with
full privileges and able to change the group of /etc/bitcoin.

1: https://github.com/systemd/systemd/blob/60b45a80c1f98bad000bd902d97ecf6c4e3fc315/NEWS#L2434

contrib/init/bitcoind.service

@@ -25,8 +25,7 @@ ExecStart=/usr/bin/bitcoind -pid=/run/bitcoind/bitcoind.pid \
                             -shutdownnotify='systemd-notify --stopping'
 
 # Make sure the config directory is readable by the service user
-PermissionsStartOnly=true
-ExecStartPre=/bin/chgrp bitcoin /etc/bitcoin
+ExecStartPre=!/bin/chgrp bitcoin /etc/bitcoin
 
 # Process management
 ####################