Update maven endpoint + Jenkinsfile + upload task + add warning

[SizableShrimp]

This is mainly for all the whiners out there. This PR is simple in what it does. I updated the maven endpoint to the new one, along with the Jenkinsfile and upload task used by Jenkins. I have no real way to test the Jenkinsfile, but I copied the one from FG4 and modified one line to use the correct gradle task for the older version. I also modified the actual upload task to use the modified variables. A warning is now spit out every time something relating to gradle is executed warning the user that the version is unsupported and they should upgrade or see the Forge discord for more info on helping. The exact message is:

WARNING: You are using an unsupported version of ForgeGradle.
Please consider upgrading to ForgeGradle 4 and helping in the efforts to get old versions working on the modern toolchain. See #mcpconfig in the Forge discord for more info.

[CDAGaming]

I can see this likely being the last warning for <1.11.2 users (As 1.12.2 still works fine in FG4). LGTM

[LexManos]

The entire point of the RetroGradle people, is to do that I did with 1.12.2, and make a MCPConfig/Update the Forge branches to build the userdevs. It is completely possible for FG4 to be used on ANY MC versions. It just takes someone putting in the effort. Which is why this is a call to people to help out. If they are so concerned about the old versions working still, they can spend some time helping upgrade the toolchain.

[LexManos]

That was not an invitation to try and justify their argument. I have heard it all before, and it is all invalid. The decision has been made on how to address this. I was simply adding context to the warning message.