Update supported platforms and EOL platform dates

[IanMadd]

Description

Update supported platforms for Client 18:

• remove Oracle Linux 6
• remove CentOS, everything but version 7
• remove macOS 11

Updated platform EOL dates at the bottom of the page.

Definition of Done

Issues Resolved

[List any existing issues this PR resolves, or any Discourse or StackOverflow discussion that's relevant]

Related PRs

Check List

• [ ] Spell Check
• [ ] Local build
• [ ] Examine the local build
• [ ] All tests pass

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[sonarcloud[bot]]

Quality Gate passed

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