[AIRFLOW-5842] Swtch to Debian buster image as a base

[potiuk]

---

Issue link: AIRFLOW-5842

Make sure to mark the boxes below before creating PR: [x]

• Description above provides context of the change
• Commit message/PR title starts with [AIRFLOW-NNNN]. AIRFLOW-NNNN = JIRA ID^*
• Unit tests coverage for changes (not needed for documentation changes)
• Commits follow "How to write a good git commit message"
• Relevant documentation is updated including usage instructions.
• I will engage committers as explained in Contribution Workflow Example.

^* For document-only changes commit message can start with [AIRFLOW-XXXX].

---

In case of fundamental code change, Airflow Improvement Proposal (AIP) is needed.
In case of a new dependency, check compliance with the ASF 3rd Party License Policy.
In case of backwards incompatible changes please leave a note in UPDATING.md.
Read the Pull Request Guidelines for more information.

[codecov-io]

Codecov Report

Merging #7647 into master will increase coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #7647      +/-   ##
==========================================
+ Coverage   86.83%   86.86%   +0.02%     
==========================================
  Files         897      897              
  Lines       42808    42808              
==========================================
+ Hits        37173    37184      +11     
+ Misses       5635     5624      -11

Impacted Files	Coverage Δ
airflow/utils/process_utils.py	86.04% <0%> (+12.79%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 680f767...f514735. Read the comment docs.