Update to Minecraft 26.1

[eclipseisoffline]

This PR updates Rainbow to Minecraft 26.1.x. This includes, but is not limited to, the following changes:

• Buildscript updates for Java 26 and deobfuscated versions of Minecraft.
• Gradle wrapper has been updated to 9.4.1.
• A lot of import/mixin fixes.
• Primarily using ItemStackTemplate in the mapping process.
• Initialising (item) data components in the datagen module.

Along with these changes, the following features/fixes have been implemented as well:

• Support for transformations in item model definitions (only translation and scale transformations, could have issues).
• A proper third_person condition in generated animation files.
• A more proper report.txt file with no FIXME counts.
• Fixed exporting of armour textures, or other textures with no atlas (Armor rendering doesnt work #21).

Rainbow's code base has also been properly null-marked, with all nullability annotations now using JSpecify's set. Lastly, publishing to Modrinth is now enabled.