Regenerate iamcredentials client (googleapis#9545)

This PR was generated using Autosynth. 🌈


<details><summary>Log from Synthtool</summary>

```
2021-08-21 03:57:13,474 synthtool [DEBUG] > Executing /home/kbuilder/.cache/synthtool/google-api-java-client-services/synth.py.
On branch autosynth-iamcredentials
nothing to commit, working tree clean
2021-08-21 03:57:13,896 synthtool [DEBUG] > Cloning https://github.com/googleapis/discovery-artifact-manager.git.
DEBUG:synthtool:Cloning https://github.com/googleapis/discovery-artifact-manager.git.
2021-08-21 03:57:14,330 synthtool [DEBUG] > Cleaning output directory.
DEBUG:synthtool:Cleaning output directory.
2021-08-21 03:57:14,332 synthtool [DEBUG] > Installing dependencies.
DEBUG:synthtool:Installing dependencies.
2021-08-21 03:57:16,032 synthtool [INFO] > Generating iamcredentials v1.
INFO:synthtool:Generating iamcredentials v1.
2021-08-21 03:57:16,032 synthtool [INFO] > 	1.31.0
INFO:synthtool:	1.31.0
Writing json metadata to clients/google-api-services-iamcredentials/v1.metadata.json
2021-08-21 03:57:16,514 synthtool [DEBUG] > Wrote metadata to clients/google-api-services-iamcredentials/synth.metadata.
DEBUG:synthtool:Wrote metadata to clients/google-api-services-iamcredentials/synth.metadata.

```
</details>

Full log will be available here:
https://source.cloud.google.com/results/invocations/9e01e508-40c5-4b4b-bb5d-c2b5c95c57ba/targets

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Author: yoshi-automation