feat(self-update): show old version in update message (#6473)
## Summary
Indicate the previous version from which uv was upgraded when running
`uv self update`. Thought that it could be useful in some situations to
have a trace of the previous version that was installed.
## Test Plan
Did not find a way to test this, since this heavily relies on being able
to use the installation script and the ability to publish artifacts for
a specific tag.
