Change version to 3.1 as preparation for release (#552)

Signed-off-by: Erik Jaegervall <erik.jaegervall@se.bosch.com>

VERSION

@@ -1 +1 @@
-3.1-develop
+3.1

docs-gen/layouts/partials/menu-footer.html

@@ -8,7 +8,7 @@
     <!-- Place this tag where you want the button to render. -->
     <a class="github-button" href="https://github.com/covesa/vehicle_signal_specification/fork" data-icon="octicon-repo-forked" data-show-count="true" aria-label="Fork matcornic/hugo-theme-learn on GitHub">Fork</a>
 
-    Latest Released Version: 3.0
+    Latest Released Version: 3.1
 </center>
 <!-- Place this tag in your head or just before your close body tag. -->
 <script async defer src="https://buttons.github.io/buttons.js"></script>

spec/Vehicle/Vehicle.vspec

@@ -41,7 +41,7 @@ VersionVSS.Patch:
 VersionVSS.Label:
   datatype: string
   type: attribute
-  default: 'develop'
+  default: ''
   description: Label to further describe the version.
 
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