Bump Ubuntu to 22.04 for readthedocs (#26)

Co-authored-by: repo-helper[bot] <74742576+repo-helper[bot]@users.noreply.github.com>

Author: repo-helper[bot]

.readthedocs.yml

@@ -13,7 +13,7 @@ python:
    - requirements: requirements.txt
    - requirements: doc-source/requirements.txt
 build:
-  os: ubuntu-20.04
+  os: ubuntu-22.04
   tools:
     python: '3.9'
   jobs: