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.

This is a cherry-pick of #3270

Check List

• Commits are signed per the DCO using --signoff

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[coveralls]

Pull Request Test Coverage Report for Build 12418803465

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 866 unchanged lines in 7 files lost coverage.
• Overall coverage increased (+0.03%) to 74.848%

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Files with Coverage Reduction	New Missed Lines	%
contrib/babelfishpg_tsql/src/pl_handler.c	48	90.09%
contrib/babelfishpg_tds/src/include/tds_request.h	53	74.85%
contrib/babelfishpg_tsql/src/pltsql_utils.c	68	92.23%
contrib/babelfishpg_tsql/runtime/functions.c	108	83.21%
contrib/babelfishpg_tsql/src/procedures.c	144	88.75%
contrib/babelfishpg_tsql/src/catalog.c	202	86.43%
contrib/babelfishpg_tsql/src/tsqlIface.cpp	243	90.11%

Totals
Change from base Build 12395755914:	0.03%
Covered Lines:	46532
Relevant Lines:	62169

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💛 - Coveralls