How to remove the last * with aws "Intrinsic Function Reference"

Serverless Framework
#1

Get the log group ARN output as

arn:aws:logs:us-east-1:123456789012:log-group:/mystack-testgroup-12ABC1AB12A1:*

with code

Outputs:
    CustomAccessLogsArn:
      Value:
        "Fn::GetAtt": [ CustomAccessLogs, Arn ]

But in fact, it is not the right ARN, so I have to remove the last part of :*

change to

arn:aws:logs:us-east-1:123456789012:log-group:/mystack-testgroup-12ABC1AB12A1

Play around with Fn::Split, Fn::Select and Fn::Sub, can’t get the expect output.

Any suggestion?