fix(s3): Notifications fail to deploy due to incompatible node runtime (#13624)

This [PR](#13488) upgraded our Node runtimes from `10` to `14`. The problem is that Node14 isn't supported for lambda functions using inline code (i.e `ZipFile`). 

Change to Node12 specifically for the notification handler since it's the only one using `InlineLambda`. 

Fixes #13620

----

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

Author: iliapolo

packages/@aws-cdk/aws-lambda-event-sources/test/integ.s3.expected.json

@@ -185,7 +185,7 @@
             "Arn"
           ]
         },
-        "Runtime": "nodejs14.x",
+        "Runtime": "nodejs12.x",
         "Timeout": 300
       },
       "DependsOn": [

packages/@aws-cdk/aws-s3-notifications/test/integ.notifications.expected.json

@@ -220,7 +220,7 @@
             "Arn"
           ]
         },
-        "Runtime": "nodejs14.x",
+        "Runtime": "nodejs12.x",
         "Timeout": 300
       },
       "DependsOn": [

packages/@aws-cdk/aws-s3-notifications/test/lambda/integ.bucket-notifications.expected.json

@@ -244,7 +244,7 @@
             "Arn"
           ]
         },
-        "Runtime": "nodejs14.x",
+        "Runtime": "nodejs12.x",
         "Timeout": 300
       },
       "DependsOn": [

packages/@aws-cdk/aws-s3-notifications/test/sns/integ.sns-bucket-notifications.expected.json

@@ -203,7 +203,7 @@
             "Arn"
           ]
         },
-        "Runtime": "nodejs14.x",
+        "Runtime": "nodejs12.x",
         "Timeout": 300
       },
       "DependsOn": [

packages/@aws-cdk/aws-s3-notifications/test/sqs/integ.bucket-notifications.expected.json

@@ -192,7 +192,7 @@
             "Arn"
           ]
         },
-        "Runtime": "nodejs14.x",
+        "Runtime": "nodejs12.x",
         "Timeout": 300
       },
       "DependsOn": [

packages/@aws-cdk/aws-s3/lib/notifications-resource/notifications-resource-handler.ts

@@ -82,7 +82,7 @@ export class NotificationsResourceHandler extends Construct {
         Code: { ZipFile: `exports.handler = ${handler.toString()};` },
         Handler: 'index.handler',
         Role: role.roleArn,
-        Runtime: 'nodejs14.x',
+        Runtime: 'nodejs12.x',
         Timeout: 300,
       },
     });