fix pipeline building by updating the mkdocs versions

@grigoras currently pipeline is broken with error:

  File "/usr/local/lib/python3.8/site-packages/mkdocs/utils/filters.py", line 13, in <module>
    @jinja2.contextfilter
AttributeError: module 'jinja2' has no attribute 'contextfilter'

solution found: https://github.com/readthedocs/readthedocs.org/issues/9064

.gitlab-ci.yml

 variables:
   GIT_SUBMODULE_STRATEGY: recursive
-  MKDOCS_VERSION: '1.2.1'
-  MATERIAL_VERSION: '7.1.10'
-  I18NSTATIC_VERSION: '0.17'
+  MKDOCS_VERSION: '1.3.0'
+  MATERIAL_VERSION: '8.2.11'
+  MATERIAL_EXT_VERSION: '1.0.3'
+  I18NSTATIC_VERSION: '0.45'
 
 stages:
   - pages
@@ -12,9 +13,9 @@ pages:
   stage: pages
   script:
     - apk add --no-cache git subversion
-    - pip install --upgrade pip setuptools wheel
-    - pip install mkdocs==${MKDOCS_VERSION} mkdocs-material==${MATERIAL_VERSION} mkdocs-static-i18n==${I18NSTATIC_VERSION}
-    - if [[ -f requirements.txt ]]; then pip install -r requirements.txt; fi;
+    - python3 -m pip install --no-cache-dir --upgrade pip setuptools wheel
+    - if [[ -f requirements.txt ]]; then python3 -m pip install --no-cache-dir -r requirements.txt; fi;
+    - python3 -m pip install --no-cache-dir --upgrade mkdocs==${MKDOCS_VERSION} mkdocs-material==${MATERIAL_VERSION} mkdocs-material-extensions==${MATERIAL_EXT_VERSION} mkdocs-static-i18n==${I18NSTATIC_VERSION}
     - mkdocs build --strict -d public
     - if [[ -f _redirects ]]; then cp _redirects public; fi;
   artifacts:
@@ -25,4 +26,3 @@ pages:
     - if: '$CI_COMMIT_BRANCH == "master"'
       when: always
 
-