docs: update 6.5.0 changelog

[janoshrubos]

PR Checklist

• The PR title follows our guidelines: https://github.com/NativeScript/NativeScript/blob/master/CONTRIBUTING.md#commit-messages.
• There is an issue for the bug/feature this PR is for. To avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it.
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• Tests for the changes are included.

What is the current behavior?

Changelog for 6.5.0 is missing, the changes are in 6.4.1.

What is the new behavior?

Changelog for 6.5.0 is fixed.

Fixes #5312.