Fix Gel cardinality warnings (#3425)

Author: CarsonF

dbschema/engagement.gel

@@ -60,7 +60,7 @@ module default {
     );
     trigger denyDuplicateFirstScriptureBasedOnOtherEngagement after insert, update for each do (
       assert( 
-        not exists (select __new__.language.engagements filter .firstScripture),
+        not exists (select __new__.language.engagements filter assert_single(.firstScripture)),
         message := "Another engagement has already been marked as having done the first scripture"
       )
     );

dbschema/language.gel

@@ -87,7 +87,7 @@ module default {
       select LanguageEngagement filter __source__ = .language
     );
     projects := (
-      select TranslationProject filter __source__ = .languages
+      select TranslationProject filter __source__ in .languages
     );
     overloaded link projectContext: Project::Context {
       default := (insert Project::Context);

dbschema/migrations/00020-m17rl5l.edgeql

@@ -43,11 +43,11 @@ with
       set {
         pins += (
           select Mixin::Pinnable
-          filter [is Mixin::Named].name = <str>json_array_unpack(user['pinnables'])
+          filter [is Mixin::Named].name in <str>json_array_unpack(user['pinnables'])
         )
       }
     )
   ),
   modified := (select users filter .modifiedAt = datetime_of_transaction())
-select {`Modified Users` := modified.realFirstName ++ ' ' ++ modified.realLastName}
+select {`Added Pins to Users` := modified.realFirstName ++ ' ' ++ modified.realLastName}
 filter count(modified) > 0;