[1.21.3] Append unknown modded flags to `removed_features` level data

[ApexModder]

In vanilla Mojang has data fixers which appends all legacy/removed feature flags to a list (removed_features) in the level data, allowing crash reports to be aware of which flags were active and have since been removed.

This PR implements a simple hook to append unknown modded flags to this list, allowing modders to know if a crash happened in an environment where their alpha testing was once enabled.

An example crash report can be found here, take note of the Removed feature flags: entry, which lists all our test flags.

[neoforged-pr-publishing[bot]]

• [x] Publish PR to GitHub Packages

Last commit published: 3bb85857dddd441356f4b453557f84c54f76dc04.

PR Publishing

### The artifacts published by this PR: - :package: [`net.neoforged:testframework:21.3.42-beta-pr-1703-pr-removed-flags`](https://github.com/neoforged/NeoForge/packages/2324270) - :package: [`net.neoforged:neoforge:21.3.42-beta-pr-1703-pr-removed-flags`](https://github.com/neoforged/NeoForge/packages/2324269) ### Repository Declaration In order to use the artifacts published by the PR, add the following repository to your buildscript: ```gradle repositories { maven { name 'Maven for PR #1703' // https://github.com/neoforged/NeoForge/pull/1703 url 'https://prmaven.neoforged.net/NeoForge/pr1703' content { includeModule('net.neoforged', 'testframework') includeModule('net.neoforged', 'neoforge') } } } ``` ### MDK installation In order to setup a MDK using the latest PR version, run the following commands in a terminal. The script works on both *nix and Windows as long as you have the JDK `bin` folder on the path. The script will clone the MDK in a folder named `NeoForge-pr1703`. On Powershell you will need to remove the `-L` flag from the `curl` invocation. ```sh mkdir NeoForge-pr1703 cd NeoForge-pr1703 curl -L https://prmaven.neoforged.net/NeoForge/pr1703/net/neoforged/neoforge/21.3.42-beta-pr-1703-pr-removed-flags/mdk-pr1703.zip -o mdk.zip jar xf mdk.zip rm mdk.zip || del mdk.zip ``` To test a production environment, you can download the installer from [here](https://prmaven.neoforged.net/NeoForge/pr1703/net.neoforged/neoforge/21.3.42-beta-pr-1703-pr-removed-flags/neoforge-21.3.42-beta-pr-1703-pr-removed-flags-installer.jar).

[neoforged-releases[bot]]

🚀 This PR has been released as NeoForge version 21.3.46-beta.