Indicate rebootstrap requirements in source-build release announcements

Source-build release announcements do not clearly state whether consumers can build using the previous source-built release or must rebootstrap.

### Proposal

Add a field to every source-build release announcement

> **[Rebootstrap](https://github.com/dotnet/source-build/blob/main/Documentation/bootstrapping-guidelines.md) required:** Yes/No

Start by locating the source-build release announcements and read Documentation/bootstrapping-guidelines.md for the rebootstrap criteria. Update every source-build release announcement to include the proposed “Rebootstrap required: Yes/No” field, then verify that each announcement states whether the previous source-built release can be used.

Written by the indexing model from the issue text.