### What changes were proposed in this pull request?
This PR proposes to fix the `Dockerfile` for Spark release.
- Port b135db3b1a to `Dockerfile`
- Upgrade Ubuntu 18.04 -> 20.04 (because of porting b135db3)
- Remove Python 2 (because of Ubuntu upgrade)
- Use built-in Python 3.8.5 (because of Ubuntu upgrade)
- Node.js 11 -> 12 (because of Ubuntu upgrade)
- Ruby 2.5 -> 2.7 (because of Ubuntu upgrade)
- Python dependencies and Jekyll + plugins upgrade to the latest as it's used in GitHub Actions build (unrelated to the issue itself)
### Why are the changes needed?
To make a Spark release :-).
### Does this PR introduce _any_ user-facing change?
No, dev-only.
### How was this patch tested?
Manually tested via:
```bash
cd dev/create-release/spark-rm
docker build -t spark-rm --build-arg UID=$UID .
```
```
...
Successfully built 516d7943634f
Successfully tagged spark-rm:latest
```
Closes#30971 from HyukjinKwon/SPARK-33927.
Lead-authored-by: Hyukjin Kwon <gurwls223@apache.org>
Co-authored-by: HyukjinKwon <gurwls223@apache.org>
Signed-off-by: HyukjinKwon <gurwls223@apache.org>