Archive old releases

Our release process indicates we should archive old releases. However, we have not yet done so, as archiving updates sites made them break.

I tested the following:

• Archived v0.7-M1 and v0.7-RC as part of the release of v0.8.
• Entered https://download.eclipse.org/escet/v0.7-M1/update-site into the Repository Explorer. The update sites loads normally, even though it is actually at https://archive.eclipse.org/escet/v0.7-M1/update-site.
• Installed SeText from the https://download.eclipse.org/escet/v0.7-M1/update-site manually into an Eclipse Modeling Tools 2021-06.

This all works. I don't see any reason to hold off on archiving old releases any longer.

We should:

• Update the release process documentation.
• Archive old releases as per the release process.