docs: Upgrade HTTP urls to HTTPS in documentation

[sakethpanuganti]

Update insecure http:// links to https:// across Markdown documentation
files. All referenced sites support HTTPS.

• multirelease.md: openjdk.java.net → openjdk.org (permanent redirect)

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[github-actions]

@desruisseaux Please assign appropriate label to PR according to the type of change.