chore: use camelcase for STS client methods

Andy Fowlstonrequested to merge
issue/aws-3609514:3609514-get-to-phpstan into 2.0.x

This matches the method names declared in the source via @method. It's safe to change a call on the STS client from AssumeRole to assumeRole: \Aws\AwsClientTrait::__call() calls \Aws\AwsClient::getCommand() which executes ucfirst($name) on the method name before calling it.

Addresses: https://www.drupal.org/project/aws/issues/3609514

Closes #3609514

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