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

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:

https://github.com/openwrt/luci/blob/74aacfbf323bf192ebaa5c15f22f3a51cdeebff2/modules/luci-mod-network/htdocs/luci-static/resources/view/network/interfaces.js#L1040