web: change 'Attributes' to 'Custom attributes' on Invitation Field (#7145)

Everywhere else, the label "Attributes" refers to the dictionary field that our
core models have and use as a JSON repository for key/value pairs that don't need
the full model's awareness or management.  Using the same label here, where the
term refers to the context data of the invitation itself, may confuse people,
so we've changed the label to signal its difference.

web/src/admin/stages/invitation/InvitationForm.ts

@@ -77,7 +77,7 @@ export class InvitationForm extends ModelForm<Invitation, string> {
                     )}
                 </p>
             </ak-form-element-horizontal>
-            <ak-form-element-horizontal label=${msg("Attributes")} name="fixedData">
+            <ak-form-element-horizontal label=${msg("Custom attributes")} name="fixedData">
                 <ak-codemirror
                     mode="yaml"
                     value="${YAML.stringify(first(this.instance?.fixedData, {}))}"