chore: Remove unnecessary usage of deadsnakes ppa

[sebathomas]

Summary

Python 3.8 is the default version in Ubuntu focal, it's not necessary to get it from a ppa.

Test Plan

CI

Additional Information

• This change is backwards-breaking

[github-actions]

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[github-actions]

feg-workflow

    2 files  203 suites   38s ⏱️
371 tests 371 ✔️ 0 💤 0 ❌
385 runs  385 ✔️ 0 💤 0 ❌

Results for commit 4c9a765.

♻️ This comment has been updated with latest results.

[github-actions]

dp-workflow

18 tests   18 ✔️  4m 22s ⏱️
  2 suites    0 💤
  2 files      0 ❌

Results for commit 4c9a765.

[github-actions]

cloud-workflow

982 tests   982 ✔️  2m 12s ⏱️
355 suites      0 💤
    7 files        0 ❌

Results for commit 4c9a765.

[github-actions]

agw-workflow

     79 files     124 suites   6m 41s ⏱️
1 154 tests 1 150 ✔️ 4 💤 0 ❌
1 156 runs  1 152 ✔️ 4 💤 0 ❌

Results for commit 4c9a765.

[maxhbr]

LGTM