Remove all sourcegraph/sourcegraph PR links from the technical changelog (#846)

The sourcegraph/sourcegraph repo is now private, so all links to it are
404 for anyone not logged in to GitHub as a Sourcegraph employee.
Knowing the PR is useful for Sourcegraph internally, but including a
link that doesn't resolve is not useful for customers, and possibly
detrimental.

The format of the PR non-link we settled on is `(PR #1234)`

Before:

- remove 5.10 deprecation dates on out of band migrations
[#1996](sourcegraph/sourcegraph#1996)

After:

- remove 5.10 deprecation dates on out of band migrations `(PR #1996)`

Author: peterguy