docs: Removed unusable devctr guide

From: Jonathan Woollett-Light <jcawl@amazon.co.uk>

Removed out of date guide (`devctr-image.md`) for updating the dev
container image as the guide could not be used by non-maintainers.

Update the link from the 0.22.0 changelog to point towards the 0.22.0
branch.

Signed-off-by: Jonathan Woollett-Light <jcawl@amazon.co.uk>
Co-authored-by: Patrick Roy <roypat@amazon.co.uk>
Signed-off-by: Patrick Roy <roypat@amazon.co.uk>

CHANGELOG.md

@@ -817,7 +817,9 @@ and this project adheres to
   `--show-level` and `--show-log-origin` that can be used for configuring the
   Logger when starting the process. When using this method for configuration,
   only `--log-path` is mandatory.
-- Added a [guide](docs/devctr-image.md) for updating the dev container image.
+- Added a
+  [guide](https://github.com/firecracker-microvm/firecracker/blob/v0.22.0/docs/devctr-image.md)
+  for updating the dev container image.
 - Added a new API call, `PUT /mmds/config`, for configuring the `MMDS` with a
   custom valid link-local IPv4 address.
 - Added experimental JSON response format support for MMDS guest applications