update "making release" page following 6.4 release

[pdurbin]

We have a thread going in Slack about a new version of https://guides.dataverse.org/en/6.4/developers/making-releases.html that includes stuff like:

• performance testing
• smoke testing
• deploying a release candidate to demo

Also @cmbz and I noticed that the person making releases no longer needs to create GitHub issues, so a bit of cleanup can be done there.

Other thoughts:

• at the top, clarify if this will be a regular or hotfix release
• declare a freeze on merging to develop

I'll make a PR and we can hash out the details there.