[discord] add a placeholder for the server start time

[RewardedIvan]

Information

This PR does not close any issues.

Details

Proposed feature:

add a "server start time (in seconds)" template in the server start message

Environments tested:

OS: Windows 10

Java version: openjdk 11.0.22 2024-01-16

• [x] Most recent Paper version (1.20.4, git-Paper-430)
• [ ] CraftBukkit/Spigot/Paper 1.12.2
• [ ] CraftBukkit 1.8.8

Demonstration: