Force to use pip<24 for Plone 5.2

```
WARNING: Ignoring version 5.2.5 of Products.CMFPlone since it has invalid metadata:
Requested Products.CMFPlone from https://files.pythonhosted.org/packages/a8/b1/7f746d0ecf0fc95c3e81585aa4f6dddca04be643509aa803754d736ab7e6/Products.CMFPlone-5.2.5-py2.py3-none-any.whl (from plone.app.contentmenu==2.3.3->-r requirements-mxdev.txt (line 8)) has invalid metadata: Expected matching RIGHT_PARENTHESIS for LEFT_PARENTHESIS, after version specifier
    plone.app.contentmenu (>=1.1.6dev-r22380)
                          ~~~~~~~~~~~~^
Please use pip<24.1 if you need to use this version.
```

In connection with this, avoid using the setup-plone action because it's
unclear how it can be forced to use a specific old version of pip.

.github/workflows/test-plone-5.2.yml

@@ -29,10 +29,12 @@ jobs:
 
       # python setup
       - name: Set up Python ${{ matrix.python-version }} with Plone 5.2.5
-        uses: plone/[email protected]
-        with:
-          python-version: ${{ matrix.python-version }}
-          plone-version: "5.2.5"
+        run: |
+          make install-plone-5.2
+
+        ## Note: setup-plone action is retired, because we need to force
+        ## an older pip version, but the action seems to fetch the new version
+        ## regardless what I try.
 
       # python cache
       - uses: actions/cache@v1

Makefile

@@ -106,6 +106,7 @@ install-plone-5.2: bin/mxdev config ## pip install Plone packages
 	@echo "$(GREEN)==> Setup Build$(RESET)"
 	cp constraints-5.2.txt constraints.txt
 	cp requirements-5.2.txt requirements.txt
+	pip install pip\<24
 	bin/tox -e init
 	bin/mxdev -c mx.ini
 	bin/pip install -r requirements-mxdev.txt

requirements-5.2.txt

@@ -1,4 +1,5 @@
 -c constraints.txt
+pip<24
 
 # Cannot install new testing framework with 5.2
 # -e ".[test]"