syslog-ng: bump version header in conf file to 3.10

syslog-ng: bump version header in conf file to 3.10

Remove a runtime warning message about configuration file being too old.

Do the same as commit 3dad25466d "syslog-ng: Bump version header in conf
file to 3.9". Package version of syslog-ng is 3.10.1, so bump version
number in syslog-ng.conf to 3.10.

Also add a comment to avoid the same warning message reappears when the
package is bumped.

Signed-off-by: Ricardo Martincoski <ricardo.martincoski@datacom.ind.br>
Cc: Chris Packham <judge.packham@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>