Pin platformio to verison 6.1.6.

Version 6.1.7 was released on 2023-05-08 and all CI runs are failing with:

```
Error: Not a PlatformIO project. `platformio.ini` file has not been found in current working directory (examples/PIO_TestPatterns). To initialize new project please use `platformio project init` command
```

The new release notes are here: https://docs.platformio.org/en/latest/core/history.html#id2. Perhaps this breakage has something to do with new validation that `project working environment names ... only contain lowercase letters a-z, numbers 0-9, and special characters _ (underscore) and - (hyphen)` while this repo uses an example named `PIO_TestPatterns`?

Let's pin ourselves to a known good version so we can fix the issue and then choose when to upgrade in the future without causing unexpected breakages.

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