fix(version): harden java version check by using Runtime class

[FredPraca]

On Manjaro, using java.version returns more than two digits in version number.
Using strJVMVersion.lastIndexOf('.') to get the major version as substring returns a not parseable number triggering NumberFormatException.
For example, I currently run openjdk 21.0.12.1 2026-08-18, so the current algorithm returns 21.0.12 as major version.

Using Runtime.version() is more stable and provide all version numbers already typed.

[AtesComp]

Look eminently reasonable and straight forward to me.

[AtesComp]

I'll need to package an updated release, but at least it's in the code. I do see the irony in doing a version release for proper version checking.

[FredPraca]

I didn't see it this way but you're right, that's quite ironic 😁
But you merged it so fast, that nobody will remember this 😉