Proposal revisions for SE-0461. (#2751)

* [SE-0461] Rename `@execution(concurrent)` to `@concurrent`.

* [SE-0461] Remove the migration-related warning in favor of automatic migration
tooling.

* [SE-0461] Add a section about removing `noasync` from the `assumeIsolated` API
family.

* [SE-0461] Add a section about region isolation rules.

* [SE-0461] Update table of contents.

Author: hborla