odhcpd: Cap ra_lifetime to 5400s and update LuCI text prompt

[hyfink]

In the latest version of odhcp(2026.06.29~5d7be43f-r1), the ra_lifetime parameter has been capped at a maximum of 5400 seconds. The text prompt in the LuCI interface needs to be updated. The code is located at:

luci/modules/luci-mod-network/htdocs/luci-static/resources/view/network/interfaces.js

Line 1040 in 74aacfb

so = ss.taboption('ipv6-ra', form.Value, 'ra_lifetime', _('<abbr title="Router Advertisement">RA</abbr> Lifetime'), _('Router Lifetime published in <abbr title="Router Advertisement, ICMPv6 Type 134">RA</abbr> messages. Maximum is 9000 seconds.'));