protoc 21.7 not found

I originally thought this was a typo, but it seems protoc 21.7 does exist. So, edit:

The [build.gradle snippet in the web tutorial](https://developer.android.com/codelabs/android-proto-datastore#4) specifies:

```
protoc {
artifact = "com.google.protobuf:protoc:21.7"
}
```

But Android Studio can't find protoc 21.7 with the provided repository setting:

```
repositories {
google()
mavenCentral()
}
```
giving the error:

```
> Could not resolve all files for configuration ':app:protobufToolsLocator_protoc'.
> Could not find com.google.protobuf:protoc:21.7.
Searched in the following locations:
- https://dl.google.com/dl/android/maven2/com/google/protobuf/protoc/21.7/protoc-21.7.pom
- https://repo.maven.apache.org/maven2/com/google/protobuf/protoc/21.7/protoc-21.7.pom
Required by:
project :app
```

Changing the protoc stanza to read ```artifact = "com.google.protobuf:protoc:3.21.7"``` (i.e. changing the version from 21.7 to 3.21.7) makes this problem go away.

Open the Android Proto DataStore web tutorial at the linked build.gradle snippet and compare its protoc artifact version with the reported Maven resolution error. Update the tutorial so the artifact uses the resolvable version, then verify that the displayed repository settings and Gradle configuration are consistent.

Written by the indexing model from the issue text.