Update SPDX references to GPL license with classpath exception

[AaronAtDuo]

Description

In most of the source files, the SPDX license identifier is GPL-2.0-with-classpath-exception but per https://spdx.org/licenses/GPL-2.0-with-classpath-exception.html this identifier is deprecated. I believe it should be replaced with GPL-2.0-only WITH Classpath-exception-2.0

Expected Behavior

SPDX license identifier meets current SPDX standard

Actual Behavior

Uses a deprecated identifier

Steps to Reproduce

N/A

Workarounds

N/A