docs: Mention GL_ARB_ES3_1_compatibility in release notes.

Ilia reminded me that I forgot this.
diff --git a/docs/relnotes/12.1.0.html b/docs/relnotes/12.1.0.html
index 3f7b196..ed8e5bc 100644
--- a/docs/relnotes/12.1.0.html
+++ b/docs/relnotes/12.1.0.html
@@ -45,6 +45,7 @@
 
 <ul>
 <li>GL_ARB_shader_group_vote on nvc0</li>
+<li>GL_ARB_ES3_1_compatibility on i965</li>
 </ul>
 
 <h2>Bug fixes</h2>