Some release notes improvement.

- Depersonalize (remove use of “we” etc.)
- Condense performance improvement bullet points
- Minor wording improvements.
Tweak to notes about test output per case.
Some tweaks to notes on implicit wrapper task.
Tweaks to build setup plugin notes.
Tweaks to notes on duplicates strategy.
Remove Closure overloads of CopySpec methods now that they are enhanced.
Don't try and performance test unreleased Gradle versions.