Merge branch 'develop-what4-versions'. Close #461.

**Description**

`what4` has seen a new release 1.5.1, but `copilot-theorem` needs versions
strictly lower than 1.5. This is preventing `copilot` from being included in
the new updates to Debian and, by extension, Ubuntu, which will require
`what4>=1.5.1`

**Type**

- Management: update versions of dependencies.

**Additional context**

None.

**Requester**

- Scott Talbert.

**Method to check presence of bug**

The following Dockerfile checks Copilot can be installed with what4-1.5.1, in
which case it prints the message Success:

```Dockerfile
FROM ubuntu:focal

RUN apt-get update

RUN apt-get install --yes libz-dev
RUN apt-get install --yes git

RUN apt-get install --yes wget
RUN mkdir -p $HOME/.ghcup/bin
RUN wget https://downloads.haskell.org/~ghcup/0.1.19.2/x86_64-linux-ghcup-0.1.19.2 -O $HOME/.ghcup/bin/ghcup

RUN chmod a+x $HOME/.ghcup/bin/ghcup
ENV PATH=$PATH:/root/.ghcup/bin/
ENV PATH=$PATH:/root/.cabal/bin/
RUN apt-get install --yes curl
RUN apt-get install --yes gcc g++ make libgmp3-dev

SHELL ["/bin/bash", "-c"]

RUN ghcup install ghc 9.4
RUN ghcup install cabal 3.8
RUN ghcup set ghc 9.4.7
RUN cabal update

CMD git clone $REPO \
    && cd $NAME \
    && git checkout $COMMIT \
    && cabal install --lib copilot**/ \
         --constraint="what4==1.5.1" \
    && echo Success
```

Command (substitute variables based on new path after merge):
```sh
$ docker run -e "REPO=https://github.com/copilot-language/copilot" -e "NAME=copilot" -e "COMMIT=<commit_hash>" copilot-verify-461
```

**Expected result**

Running the docker image above prints the message "Success", indicating that
Copilot can be installed with `what4-1.5.1`.

**Solution implemented**

Modify `copilot-theorem`'s cabal file to accept `what4` versions `>= 1.5` and `<1.6`.

**Further notes**

None.

copilot-theorem/CHANGELOG

@@ -1,3 +1,6 @@
+2023-11-03
+        * Relax version constraint on what4. (#461)
+
 2023-09-07
         * Version bump (3.16.1). (#455)
 

copilot-theorem/copilot-theorem.cabal

@@ -61,7 +61,7 @@ library
                           , random                >= 1.1 && < 1.3
                           , transformers          >= 0.5 && < 0.7
                           , xml                   >= 1.3 && < 1.4
-                          , what4                 >= 1.3 && < 1.5
+                          , what4                 >= 1.3 && < 1.6
 
                           , copilot-core          >= 3.16.1 && < 3.17
                           , copilot-prettyprinter >= 3.16.1 && < 3.17