Switch to Yarn mappings 26.2+build.3

Loom rejected officialMojangMappings() with a non-obfuscated
environment error, and Mojang client_mappings are not published for
26.2 yet. Pin Yarn at 26.2+build.3 via the new yarn_mappings property
and rewrite the client entrypoint with Yarn names: MinecraftClient,
Text, Formatting, inGameHud.getChatHud(), getSession().getUsername().

build.gradle

@@ -30,7 +30,7 @@ repositories {
 
 dependencies {
     minecraft "com.mojang:minecraft:${project.minecraft_version}"
-    mappings loom.officialMojangMappings()
+    mappings "net.fabricmc:yarn:${project.yarn_mappings}:v2"
     modImplementation "net.fabricmc:fabric-loader:${project.loader_version}"
     modImplementation "net.fabricmc.fabric-api:fabric-api:${project.fabric_version}"
 }