Update Dockerfiles (versions) and update guava to resolve CVEs

[mikesir87]

Couple of small updates, including:

• Updating the Java image from amazoncorretto:17 to amazoncorretto:20
• Update guava to resolve CVE
• Use /usr/local/app for all working directories, rather than / for consistency

Will do more updates later to include cache mounts in the build, run as non-root, etc. Just a few quick and easy updates right now.

[github-actions]

Docker image tag(s) pushed:

ghcr.io/dockersamples/wordsmith-api:pr-46
dockersamples/wordsmith-api:pr-46
dockersamples/k8s-wordsmith-api:pr-46

Labels added to images:

org.opencontainers.image.created=2023-09-15T21:26:13.848Z
org.opencontainers.image.description=Sample project with Docker containers running under Kubernetes
org.opencontainers.image.licenses=Apache-2.0
org.opencontainers.image.revision=4f532146780374748eac0a98eae38cf3e3d28876
org.opencontainers.image.source=https://github.com/dockersamples/wordsmith
org.opencontainers.image.title=wordsmith
org.opencontainers.image.url=https://github.com/dockersamples/wordsmith
org.opencontainers.image.version=pr-46