Use Miniforge in place of Mambaforge

Mambaforge is being retired: https://conda-forge.org/news/2024/07/29/sunsetting-mambaforge/.

Author: mdpiper

.github/workflows/test.yml

@@ -27,7 +27,6 @@ jobs:
 
       - uses: conda-incubator/setup-miniconda@v3
         with:
-          miniforge-variant: Mambaforge
           miniforge-version: latest
           python-version: ${{ matrix.python-version }}
 
@@ -38,7 +37,7 @@ jobs:
 
       - name: Install requirements
         run: |
-          mamba install --file=requirements.txt --file=requirements-testing.txt
+          conda install --file=requirements.txt --file=requirements-testing.txt
 
       - name: Build and install package
         run: pip install -e .