`MultiSourceReader` has breaking behavioural changes when running on Java 16+

[jord1e]

Describe the bug Because of JDK-8241020 "LineNumberReader.getLineNumber() behavior is inconsistent with respect to EOF", the MultiSourceReader class behaves incorrectly when running on Java 16+ VMs, even when compiling with Java 8. This causes breaking changes.

The following test cases are failing:

To Reproduce

Run the tests on JDK 16, or higher Same issue occurs when you compile on JDK 8 and run tests on JDK 16+ (see results below)

For ease of testing you may temporarily clone https://github.com/jord1e/graphql-java/tree/ci-improvements and change the versions in gradle.properties (or checkout https://github.com/jord1e/graphql-java/commit/a1f4f8455c21a1beb60d38d2baff6dabf93be719)

Results can be downloaded here: https://github.com/jord1e/graphql-java/actions/runs/1778419182

Found whilst working on #2676

Can't figure this one out on myself quickly...

[github-actions[bot]]

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[jord1e]

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