fix: reset nickname when player changes Minecraft account name

[hudsonandres]

Information

This PR fixes #6469.

Details

Proposed fix:

When a premium player changes their Minecraft username, any previously set nickname (including RGB-formatted ones) is now automatically cleared on their next login, equivalent to running /nick off.

Environments tested:

OS: Linux
Java version: OpenJDK 21.0.7

Bukkit Version - Purpur 1.21.11-R0.1-SNAPSHOT

Demonstration:

Before: No log to show
After: No log to show

[hudsonandres]

How it works:

Every time a player joins, the code already compares lastAccountName (the name stored from their last login) with the current Minecraft account name.

If they differ, meaning the player changed their Minecraft username - and the player has a nickname set, setNickname(null) is called to remove it (equivalent to /nick off).

The subsequent user.setDisplayNick() call then updates their display name to the bare new account name.
This way, RGB-colored or any other nicknames tied to the old account name are automatically cleared when the player first joins after a name change.