Old Gradle version can't find Java 11

### [REQUIRED] Please fill in the following fields:

* Pre-built SDK from the [website](https://firebase.google.com/download/cpp) or open-source from this repo: open-source
* Firebase C++ SDK version: b09efc2
* Problematic Firebase Component: Android
* Other Firebase Components in use: _____ (Auth, Database, etc.)
* Platform you are using the C++ SDK on: Linux
* Platform you are targeting: Android

### [REQUIRED] Please describe the issue here:

I'm trying to build the Android SDK from source. I'm using Fedora 33 with OpenJDK 11.0.9.1. Running the gradle wrapper doesn't work (no matter the target):

```
$ ./gradlew :admob:assembleRelease

FAILURE: Build failed with an exception.

* What went wrong:
Could not determine java version from '11.0.9.1'.

* Try:
Run with --stacktrace option to get the stack trace. Run with --info or --debug option to get more log output. Run with --scan to get full insights.

* Get more help at https://help.gradle.org
```

#### Steps to reproduce:

Have you been able to reproduce this issue with just the [Firebase C++ quickstarts](https://github.com/firebase/quickstart-cpp) ? no
What's the issue repro rate? 100%

What happened? How can we make the problem occur?
See above.

This would be fixed by updating to a newer Gradle version.