Add support for 1.21

[CyR1en]

This PR contains code to support 1.21 servers.

Test Build Environment:

• JRE/JDK 21
• Successfully built and tested with a CommandPrompter upcoming update.

Notes for review:

• The majority of the code is similar to 1_20_R4.
• World accessor for EntityPlayer changed from dP() -> cN().
• Version bumped to 2.3.4 for all modules.
• Updated README.md

[Rapha149]

Looks good, thank you for contributing! I made one little change, I replaced Entity#cN with Entity#dO (p.cN() → p.dO()). I don't think it has an impact, the method Entity#cN points to the method Entity#dO but in the 1_20_R4 wrapper I used the method Entity#dP which looks like this:

public World dP() {
    return this.r;
}

The method in Entity#dO in 1_21_R1 has exactly the same content:

public World dO() {
    return this.r;
}

while the method Entity#cN points to the other method:

public World cN() {
    return this.dO();
}

So it doesn't really change anything, but it now uses the "same" method as in 1_20_R4.

[Rapha149]

The update is now published on Maven Central.