Go 1.21 Upgrade

Fixes #11351

tpl/internal/go_templates/htmltemplate/attr_string.go

@@ -4,6 +4,18 @@ package template
 
 import "strconv"
 
+func _() {
+	// An "invalid array index" compiler error signifies that the constant values have changed.
+	// Re-run the stringer command to generate them again.
+	var x [1]struct{}
+	_ = x[attrNone-0]
+	_ = x[attrScript-1]
+	_ = x[attrScriptType-2]
+	_ = x[attrStyle-3]
+	_ = x[attrURL-4]
+	_ = x[attrSrcset-5]
+}
+
 const _attr_name = "attrNoneattrScriptattrScriptTypeattrStyleattrURLattrSrcset"
 
 var _attr_index = [...]uint8{0, 8, 18, 32, 41, 48, 58}