Add changelog

Author: Kneelawk

changelogs/changelog-v2.0.0+1.21.md

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+Changes:
+
+* Same as version 2.0.0-beta.1.
+* Updated to Minecraft 1.21.
+* Divided GraphLib up into modules:
+    * `core` - Houses the primary GraphLib stuff, like graphs, nodes, and entities.
+    * `debugrender` - Houses the debug renderer.
+    * `syncing-core` - Houses the client-side graph stuff, allowing syncing implementations to transmit graphs to the
+      client.
+    * `syncing-knet` - Houses the KNet-based synchronization mechanism.
+    * `syncing-lns` - Houses the LibNetworkStack-based synchronization mechanism.
+* Made GraphLib multi-platform, supporting both Fabric and NeoForge, supporting both vanilla-gradle and
+  architectury-based setups.
+* Added Codextra library dependency.
+* Moved everything over to using `Codec`s for encoding and decoding.
+* Moved syncing over to using `StreamCodec`s for buffer encoding and decoding.
+* Removed `LEGACY_UNIVERSE` default graph universe from pre-1.0 versions.
+* Moved events system over to using Common Events library.