ci: Pin Ubuntu release to 20.04 LTS

[abhidg]

Now that 22.04 LTS has been released, at some point ubuntu-latest
will point to ubuntu-22.04 which can cause unplanned breakage,
pinning CI to 20.04 until we have a migration plan.

Additionally, drop main-eu builds as EU migration is complete.

[codecov-commenter]

Codecov Report

Merging #2686 (4539ac6) into main (a6f5b73) will increase coverage by 3.57%.
The diff coverage is 98.27%.

@@            Coverage Diff             @@
##             main    #2686      +/-   ##
==========================================
+ Coverage   62.15%   65.73%   +3.57%     
==========================================
  Files          17      146     +129     
  Lines        1197     5612    +4415     
  Branches      189     1497    +1308     
==========================================
+ Hits          744     3689    +2945     
- Misses        453     1923    +1470     

Impacted Files	Coverage Δ
...ata-serving/data-service/src/model/demographics.ts	100.00% <ø> (ø)
...ation/curator-service/api/src/util/validate-env.ts	100.00% <ø> (ø)
data-serving/data-service/src/controllers/case.ts	82.78% <97.87%> (ø)
data-serving/data-service/src/model/age-bucket.ts	100.00% <100.00%> (ø)
data-serving/data-service/src/model/case.ts	98.36% <100.00%> (ø)
data-serving/data-service/src/util/case.ts	91.01% <100.00%> (ø)
...ation/curator-service/api/src/controllers/cases.ts	48.17% <100.00%> (+0.38%)	⬆️
verification/curator-service/api/src/index.ts	88.54% <100.00%> (ø)
...nts/new-case-form-fields/PreexistingConditions.tsx	75.00% <0.00%> (ø)
...urator-service/ui/src/components/Sidebar/index.tsx	90.32% <0.00%> (ø)
... and 126 more

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[jim-sheldon]

Good stuff. We may even want to move to Alpine and/or scratch.