Add Java 11 support in v2

[MarkAckert]

Describe your pull request here:

This PR explicitly adds Java 11 as a supported distribution for Zowe v2.18.x releases going forward. This aligns with our Java 11 tests for v2.x code in zowe-install-packaging. Related: zowe/community#1601

List the file(s) included in this PR:

systemrequirements-zos.md

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