Adjust role used by GitHub action that builds docs.

[ashovlin]

Issue #, if available: N/A

Description of changes:

1. Fixes a CI failure on #807. We recently adjusted some internal accounts where we run integration tests, so CI_AWS_ROLE_ARN is no longer valid. This reuses the CI_MAIN_TESTING_ACCOUNT_ROLE_ARN instead.
   • The doc generator uses server mode, which requires AWS credentials to pass its health check. I didn't actually see it make any requests when running locally.
2. Also upgraded the credential action while here, there was a warning about old Node versions.

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[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (8990861) 32.34% compared to head (73076dd) 61.44%. Report is 1 commits behind head on dev.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## dev #808 +/- ## =========================================== + Coverage 32.34% 61.44% +29.09% =========================================== Files 271 277 +6 Lines 10604 10659 +55 Branches 1474 1484 +10 =========================================== + Hits 3430 6549 +3119 + Misses 6885 3568 -3317 - Partials 289 542 +253 ```

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