chore: update nodejs to 20 in devcontainer and gitpod (#30424)

### Reason for this change

The contribution guide recommend the active node LTS: https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md#getting-started

### Description of changes

Update node 18 to 20 in devcontainer and gitpod and also the os version to bookworm

### Description of how you validated changes

opened devcontainer and gitpod,  checkd the node version and run command `npx lerna run build --scope=aws-cdk-lib --scope=@aws-cdk-testing/framework-integ --skip-nx-cache`

### Checklist
- [x] My code adheres to the [CONTRIBUTING GUIDE](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md) and [DESIGN GUIDELINES](https://github.com/aws/aws-cdk/blob/main/docs/DESIGN_GUIDELINES.md)

----

refs:

- #25381
- #25426

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

.devcontainer/Dockerfile

@@ -1,4 +1,4 @@
-FROM jsii/superchain:1-buster-slim-node18
+FROM jsii/superchain:1-bookworm-slim-node20
 
 USER root
 

.gitpod.yml

@@ -3,7 +3,7 @@ github:
     pullRequestsFromForks: true
     addComment: true
 
-image: public.ecr.aws/jsii/superchain:1-buster-slim-node18
+image: public.ecr.aws/jsii/superchain:1-bookworm-slim-node20
 
 tasks:
   - init: |