Use the provided.al2023 Lambda runtime

Amazon Linux 2 has received an extra year of life since the last time I
looked, but I might as well do this while I have it in mind.

Author: ahamlinman

CloudFormation.yaml

@@ -59,7 +59,7 @@ Resources:
     Type: AWS::Serverless::Function
     Properties:
       Description: A randomizer slash command for Slack, with support for saving groups and more
-      Runtime: provided.al2
+      Runtime: provided.al2023
       Architectures: [arm64]
       CodeUri:
         Bucket: !Ref CodeS3Bucket