Stop testing against Wagtail 6.4

To reduce testing matrix size. Wagtail 6.4 is not officially supported
anymore by the Wagtail team.

Author: Stormheg

tox.ini

@@ -1,7 +1,7 @@
 [tox]
 envlist =
-    py{39,310,311,312}-django42-wagtail{63,64,70,71}-{sqlite,postgres}
-    py{310,311,312,313}-django51-wagtail{63,64,70,71}-{sqlite,postgres}
+    py{39,310,311,312}-django42-wagtail{63,70,71}-{sqlite,postgres}
+    py{310,311,312,313}-django51-wagtail{63,70,71}-{sqlite,postgres}
     ; 6.3 and up support Django 5.2
     py{310,311,312,313}-django52-wagtail{63,70,71}-{sqlite,postgres}