switch AWS_SSO_SESSION_EXPIRATION to RFC3339

[synfinatic]

Fixes: #837

[codecov]

Codecov Report

Attention: Patch coverage is 88.88889% with 1 line in your changes missing coverage. Please review.

Project coverage is 80.13%. Comparing base (fbee93d) to head (8eae16e).
Report is 40 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #870      +/-   ##
==========================================
- Coverage   80.32%   80.13%   -0.19%     
==========================================
  Files          37       38       +1     
  Lines        3405     3427      +22     
==========================================
+ Hits         2735     2746      +11     
- Misses        595      606      +11     
  Partials       75       75              

Flag	Coverage Δ
unittests	80.13% <88.89%> (?)

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Files	Coverage Δ
internal/ecs/http.go	100.00% <100.00%> (ø)
internal/storage/storage.go	94.23% <87.50%> (-0.21%)	⬇️

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