Remove duplicate CAST client

Both Blake House and InFact were already using lowercase version of the
CAST client markdown and image files, i.e. `_clients/cast.md` and
`images/clients/cast.png`. So the uppercase versions of the files, i.e.
`_clients/CAST.md` and `images/clients/CAST.png`, were effectively
duplicates.

This led to the following warning messages on case-insensitive
filesystems, making it hard to work on the project on such systems:

> warning: the following paths have collided (e.g. case-sensitive paths
> on a case-insensitive filesystem) and only one from the same colliding
> group is in the working tree

In this commit, I've added the website from `CAST.md` into `cast.md`,
deleted `CAST.md` & `CAST.png`, and changed the reference in
`dot-project.md` from `CAST` to `cast`. I think this is the right thing
to do, because ideally all the markdown files should be lowercase in
case they are used in URL paths.

There are other similar issues, but I plan to address them in separate
commits.

_clients/CAST.md

----
-title: CAST
-website: https://www.wearecast.org.uk/
----

_clients/cast.md

 ---
 title: CAST
+website: https://www.wearecast.org.uk/
 ---

_coops/dot-project.md

@@ -14,7 +14,7 @@ clients:
 - British-Tinnitus-Association
 - Carefree
 - carerssupportcentre
-- CAST
+- cast
 - Charity-Digital
 - comicrelief
 - coopsUK