Update compatibility to Java 11

[charlestian23]

Description

While working on the Short Truth Table puzzle editor, I wanted to use some features that were in Java 11. However, LEGUP was still only compatible with Java 8. This pull request will make LEGUP officially compatible with Java 11.

Changes made:

• Updated source compatibility from Java 1.8 to Java 11
  • Allows programmers to use new language features from Java 11
• Updated target compatibility to Java 11
  • Forces the lowest version that LEGUP can run on to be Java 11 (because of this, I have listed this pull request as a breaking change, since some people might not have Java 11 installed on their machines)

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Enhancement (improvement to an already existing feature)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

How Has This Been Tested?

Checklist:

• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation (if applicable)
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes