Update references from version 6.0.0a4 to 6.0.0a6

Author: ksuess

docs/install/source-step-by-step.md

@@ -68,7 +68,7 @@ site-packages/
 Install Plone 6 with constrained requirements using `pip`.
 
 ```shell
-pip install Plone -c https://dist.plone.org/release/6.0.0a4/constraints.txt
+pip install Plone -c https://dist.plone.org/release/6.0.0a6/constraints.txt
 ```
 
 ````{admonition} mkwsgiinstance's minimal Zope configuration
@@ -104,7 +104,7 @@ mkwsgiinstance -u admin:admin -d .
 python3.9 -m venv venv
 source venv/bin/activate
 pip install -U pip wheel
-pip install Plone -c https://dist.plone.org/release/6.0.0a4/constraints.txt
+pip install Plone -c https://dist.plone.org/release/6.0.0a6/constraints.txt
 mkwsgiinstance -u admin:admin -d .
 ```
 ````
@@ -246,7 +246,7 @@ If you want to checkout a Plone Core package for development or just want to ove
 
 ```
 # constraints.txt with unresolvable version conflict
--c https://dist.plone.org/release/6.0.0a4/constraints.txt
+-c https://dist.plone.org/release/6.0.0a6/constraints.txt
 plone.api>=2.0.0a3
 ```
 
@@ -272,7 +272,7 @@ collective.easyform
 {file}`constraints.txt`
 
 ```ini
--c https://dist.plone.org/release/6.0.0a4/constraints.txt
+-c https://dist.plone.org/release/6.0.0a6/constraints.txt
 
 # constraints of add-ons
 collective.easyform==3.4.5

docs/install/source.md

@@ -137,7 +137,7 @@ Choose from 1, 2 [1]: 1
 requirements_out [requirements-mxdev.txt]: 
 admin_user [admin]: 
 admin_password []: admin
-plone_version [6.0.0a2]: 6.0.0a4
+plone_version [6.0.0a2]: 6.0.0a6
 listen [localhost:8080]: 
 ```