Enable the .org.matrix.msc4028.encrypted_event push rule by default (#17826)

Clients will still only see this rule if the corresponding experimental
feature, `msc4028_push_encrypted_events`, is also enabled.

This aligns the implementation with MSC4028, specifically [this
section](https://github.com/matrix-org/matrix-spec-proposals/blob/giomfo/push_encrypted_events/proposals/4028-push-all-encrypted-events-except-for-muted-rooms.md#unstable-prefix).
See https://github.com/element-hq/synapse/issues/16846 for context.

rust/src/push/base_rules.rs

@@ -81,7 +81,7 @@ pub const BASE_APPEND_OVERRIDE_RULES: &[PushRule] = &[
         ))]),
         actions: Cow::Borrowed(&[Action::Notify]),
         default: true,
-        default_enabled: false,
+        default_enabled: true,
     },
     PushRule {
         rule_id: Cow::Borrowed("global/override/.m.rule.suppress_notices"),