devcontainer dependencies are out of date

[joebowbeer]

Expected Behavior

• Dockerfile uses LTS versions of Ubuntu and NodeJS
• devcontainer does not use deprecated syntax
• npm audit report is clean
• GitHub Actions use recent versions
• 3rd-party GitHub Actions are pinned to commit hash (as per GitHub recommendation)

Actual Behavior

• Ubuntu 20.04 will reach its end-of-life (EOL) on May 31, 2025
• NodeJS 16 already reached EOL on September 11, 2023
• devcontainer uses deprecated settings syntax
• npm audit finding(s)
• GitHub Actions are using old versions
• 3rd-party GitHub Actions are not pinned to commit hash

Steps to Reproduce the Problem

See source code.