chore: Switch Docker base image to ubuntu:20.10 instead of debian:10-slim

[jannfis]

Switch base image used for building Argo CD's Docker image from debian:10-slim to ubuntu:20.04.

This change should not be user visible, but might affect users who build their own images or use custom tooling. Therefore, this change has been noted in the upgrade notes 1.8 -> 1.9.

Closes #5047
Closes #5155

Checklist:

• Either (a) I've created an enhancement proposal and discussed it with the community, (b) this is a bug fix, or (c) this does not need to be in the release notes.
• The title of the PR states what changed and the related issues number (used for the release note).
• I've updated both the CLI and UI to expose my feature, or I plan to submit a second PR with them.
• Does this PR require documentation updates?
• I've updated documentation as required by this PR.
• Optional. My organization is added to USERS.md.
• I have signed off all my commits as required by DCO
• My build is green (troubleshooting builds).

[alexmt]

LGTM

[rumstead]

trivy image docker.io/argoproj/argocd                                                                                42s 06:24:56 PM
2021-01-05T18:25:26.984-0500	WARN	You should avoid using the :latest tag as it is cached. You need to specify '--clear-cache' option when :latest image is changed
2021-01-05T18:25:55.444-0500	WARN	This OS version is not on the EOL list: ubuntu 20.10
2021-01-05T18:25:55.446-0500	INFO	Detecting Ubuntu vulnerabilities...
2021-01-05T18:25:55.447-0500	INFO	Trivy skips scanning programming language libraries because no supported file was detected
2021-01-05T18:25:55.447-0500	WARN	This OS version is no longer supported by the distribution: ubuntu 20.10
2021-01-05T18:25:55.447-0500	WARN	The vulnerability detection may be insufficient because security updates are not provided

docker.io/argoproj/argocd (ubuntu 20.10)
========================================
Total: 0 (UNKNOWN: 0, LOW: 0, MEDIUM: 0, HIGH: 0, CRITICAL: 0)

woohoo!