2.1.3 does not build on OpenJDK16

I'm sorry to report that the latest release (2.1.3) does not build with docker OpenJDK16 (`openjdk:16-jdk-alpine3.12`), attaching output from `mvn package`:

```
[ERROR] Failures:
[ERROR] CacheManagerTest.cacheManagerWithNonReadableJsonFileForKey:159
Expecting:
<"">
to start with:
<"Cache ignored file the-cache-key.json because it cannot be read.
java.nio.file.AccessDeniedException:">

[ERROR] CacheManagerTest.cacheReturnsNullWhenCachedFileCannotBeRead:148
Expecting:
<{}>
to be equal to:

but was not.
[INFO]
[ERROR] Tests run: 121, Failures: 2, Errors: 0, Skipped: 1
```