GH-46363: [CI][Packaging] Use mono from community repository on Alpine instead of from testing (#46364)

### Rationale for this change

The wheels jobs for alpine are currently failing because mono can't be found on https://dl-cdn.alpinelinux.org/alpine/edge/testing

### What changes are included in this PR?

Mono seems to be part of the community repository for alpine now:
https://pkgs.alpinelinux.org/package/edge/community/x86_64/mono

### Are these changes tested?

Yes, via archery.

### Are there any user-facing changes?

No

* GitHub Issue: #46363

Authored-by: Raúl Cumplido <raulcumplido@gmail.com>
Signed-off-by: Raúl Cumplido <raulcumplido@gmail.com>

Author: raulcd

ci/docker/python-wheel-musllinux.dockerfile

@@ -37,8 +37,9 @@ RUN apk add --no-cache \
     unzip \
     wget \
     zip
-# Add mono from testing repo because it's not in the main repo
-RUN apk add --no-cache --repository=https://dl-cdn.alpinelinux.org/alpine/edge/testing mono
+# Add mono from community repo because it's not in the main repo.
+# We will be able to use the main repo once we move to alpine 3.22 or later.
+RUN apk add --no-cache --repository=https://dl-cdn.alpinelinux.org/alpine/edge/community mono
 
 # A system Python is required for ninja and vcpkg in this Dockerfile.
 # On musllinux_1_2 a system python is installed (3.12) but pip is not