Remove Ubuntu 20.04 LTS from active references

[spham-amzn]

Change summary

Remove documentation support for Ubuntu 20.04 LTS, making Ubuntu 22.04 LTS as the primary supported version.

Fixes https://github.com/o3de/o3de.org/issues/2509

Submission Checklist:

• [X] Descriptive active voice - Do descriptive sentences have a clear subject and action verb?
• [X] Answer the question at hand - Does the documentation answer a what, why, how, or where type of question?
• [X] Consistency - Does the content consistently follow the Style Guide?
• [X] Help the user - Does the documentation show the user something meaningful?