Fix compile errors on JDK 21 on Windows

[peter277]

• Update required Java to 8 (targeting 7 not supported anymore)
• Upgrade to JUnit 5 and in particular change ConvertorTest to use assertLinesMatch to fix line ending mismatch errors
• Change Convertor to use BufferedInputStream and fix charset name capitalisation issue (originally written as "UtF-8")
• Fix Windows path prefix problems in readFile function when running unit tests