Update version number to 4.2.1

Author: jngrad

CMakeLists.txt

@@ -47,7 +47,7 @@ if(POLICY CMP0074)
   cmake_policy(SET CMP0074 NEW)
 endif()
 
-set(PROJECT_VERSION "4.2.0")
+set(PROJECT_VERSION "4.2.1")
 
 #
 # CMake internal vars

doc/sphinx/introduction.rst

@@ -640,7 +640,7 @@ You may also provide the patch level, when relevant. If you developed code
 for |es| and made it available in a publicly accessible repository, you
 should consider providing the corresponding URL, for example in a footnote:
 
-    The method was implemented for ESPResSo 4.2.0[24] and the source code is
+    The method was implemented for ESPResSo 4.2.1[24] and the source code is
     available online\ :superscript:`note 1`.
 
     | ____________