aws: sts_credentials: make STS response parsing less brittle by checking for the full XML end node

[PettitWesley]

…ing for the full XML end node

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[PettitWesley]

@edsiper potentially important bug fix, which I've moved to a PR here since I messed up the branch name and PR title the first time around: #7357

This is a cherry-pick. Tested commit here: #7313 (comment)

[PettitWesley]

The branch name here is wrong, this is about response parsing not ARN parsing, oops. The commit message is correct.