Paper 1.21.6 Update

[Absolutionism]

Problem

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Solution

Updates build.gradle and gradle.properties by changing 1.21.5 -> 1.21.6 Adds paper-1.21.6.json for the testing environment Updates default.lang for new entries in enums/registries

Testing Completed

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Supporting Information

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Completes: none Related: none

[Absolutionism]

maybe we should remove the 1.21.3 environment (possibly .4 too)

Shane had mentioned removing some environments in the PR for updating to 1.21.5 and Sovde said:

Since we have between a third and a half of our users running 1.20.0/1/2/3/4/5 and 1.21.0/1/3, I think it's worth keeping them to see if we break anything. We're not obligated to fix it, but it's often worth fixing regardless and I'd like to have the warning the tests provide. We may want to consider dropping 1.19.4 for 2.12, though.

[APickledWalrus]

1.21.3 is only a couple hundred servers / few percent, but I'm fine keeping it for now