Update README to use newer Java version

[timchang514]

Description

Updating the README to use a newer Java version. The Antlr version update requires Java 8 at minimum to build.

For Ubuntu, I verified the version openjdk-21-jre was available by using a clean Ubuntu install.

For the RHEL version we typically develop on, the default version of Java available is openjdk-17, so that doesn't need changing.

Check List

• Commits are signed per the DCO using --signoff

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[coveralls]

Pull Request Test Coverage Report for Build 12383505376

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 74.834%

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Totals
Change from base Build 12375848385:	0.0%
Covered Lines:	46606
Relevant Lines:	62279

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💛 - Coveralls