Use dd-sts instead of vault because it is more robust

[xlamorlette-datadog]

The code coverage CI jobs used to fail quite often, as follows (example):

DD_API_KEY=$(vault kv get -field=key kv/k8s/gitlab-runner/nginx-datadog/datadoghq-api-key 2>/dev/null) npx --yes @datadog/datadog-ci@5.18.0 coverage upload --format=lcov .musl-build/coverage.lcov
Failed getting commits to exclude: RequestError: Request failed with status code 401

We now use [dd-sts[(https://datadoghq.atlassian.net/wiki/x/KwTmVwE) instead of vault because it should be much more reliable.

Reference documentation:

• dd-sts User guide
• ID tokens reference for Datadog services

[datadog-prod-us1-6]

🎉 All green!

🧪 All tests passed
❄️ No new flaky tests detected

🎯 Code Coverage (details)
• Patch Coverage: 100.00%
• Overall Coverage: 68.51% (+0.00%)

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