Ensure cache is clear before upgrading

If we changed some namespace / classes, we need to refresh the cache before anything else.

docs/de/user/upgrade-2.0.x-2.0.y.rst

@@ -8,6 +8,7 @@ Das neueste Release ist auf https://www.wallabag.org/pages/download-wallabag.htm
 
 ::
 
+    rm -rf var/cache/*
     git fetch origin
     git fetch --tags
     git checkout 2.0.8

docs/de/user/upgrade-2.0.x-2.1.y.rst

@@ -11,6 +11,7 @@ Das neueste Release ist auf https://www.wallabag.org/pages/download-wallabag.htm
 
 ::
 
+    rm -rf var/cache/*
     git fetch origin
     git fetch --tags
     git checkout 2.1.0

docs/en/user/upgrade-2.0.x-2.0.y.rst

@@ -8,6 +8,7 @@ The last release is published on https://www.wallabag.org/pages/download-wallaba
 
 ::
 
+    rm -rf var/cache/*
     git fetch origin
     git fetch --tags
     git checkout 2.0.8

docs/en/user/upgrade-2.0.x-2.1.y.rst

@@ -11,6 +11,7 @@ The last release is published on https://www.wallabag.org/pages/download-wallaba
 
 ::
 
+    rm -rf var/cache/*
     git fetch origin
     git fetch --tags
     git checkout 2.1.0

docs/fr/user/upgrade-2.0.x-2.0.y.rst

@@ -8,6 +8,7 @@ La dernière version de wallabag est publiée à cette adresse : https://www.wal
 
 ::
 
+    rm -rf var/cache/*
     git fetch origin
     git fetch --tags
     git checkout 2.0.8

docs/fr/user/upgrade-2.0.x-2.1.y.rst

@@ -11,6 +11,7 @@ La dernière version de wallabag est publiée à cette adresse : https://www.wal
 
 ::
 
+    rm -rf var/cache/*
     git fetch origin
     git fetch --tags
     git checkout 2.1.0