luci-mod-system: Fix cron log level settings in 23.05

luci-mod-system: Fix cron log level settings in 23.05

Busybox crond only uses level 5, 7 and 8. (And 7 and 8 only differ
on one error regarding finding a crontab without user.)

For simplicity, just provide 5 and 7 as alternatives in LuCI.
Add "Disabled" with value 9 as a choice for cron log level.

9 actually exceeds the highest built-in value 8, but can be
used to quieten the logging in cases where there are e.g. cron
jobs running each minute, which would create log spam.

Show level 7 as the default and as the first offered choice.

Add missing semicolons to the file.

Signed-off-by: Hannu Nyman <[email protected]>
(cherry picked from commit 1e7ce8de990e0bcb36b77bdb846b644aa1724568)
(cherry picked from commit ef167782ffc6b700f133557a9196ac846edb3399)
[also modernize system log size placeholder value]