Closes #172 update BYOA deployment

[jebbens]

Issue #, if available:
#172

Description of changes:
Update Python 3.9 to 3.11; streamline deployment to only use CodePipe…ine and remove unnecessary artifacts from previous CloudFormation deployment; update buildSpecs to node 18; make cdk bootstrap a prereq; update README

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[jebbens]

I just discovered some Lambda functions that are still being created using Python 3.9, even thought 3.11 is specified; some of these are created with private methods. It's possible the version is being overridden by older aws-cdk packages. I'm going to try to update the aws-cdk packages to see if that fixes it.

SOLUTION: By default, code for the CodeBuild job that deploys the PetAdoptions application is pulled from main in the upstream repo, so changes in local fork were not being used. There was already an option to specify a different branch in the upstream repo, but that doesn't help anybody working on features in a fork. So, I added an option to specify the GitLab URL. The only disadvantage of this approach is that it requires committing and pushing to the remote for the deployment before being able to test. A future PR could add uploading the local repo to S3 and some conditional logic for where to retrieve code.