6.6 Release notes

[andrew-ronaldson]

Create release highlights for PF 6.6.

@mcoker
@andrew-ronaldson
@kmcfaul
@dlabaj

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Jira Issue: PF-4335

[mcoker]

Looks like @phcox put up a PR for the 6.6 release highlights - #5045

Do we still need this issue?

[dlabaj]

Closing this issue release is out.