[OpenJDK 2D-Dev] RFR: [JDK12] 8218020 Fix version number in mesa.md 3rd party legal file

[Phil Race]

Bug : https://bugs.openjdk.java.net/browse/JDK-8218020

This is a trivial fix to a license file for JDK 12 needed because v5.0 
is what
we are tracking as using, but the license file isn't updated from v4.1
The licensed header file (just one file !) is identical between v4.1 + 
v5.0 - ie
the source file in Mesa 5.0 claims to still be v4.1 - which is probably 
why we have a mismatch

So this is purely a license file change

Diff :

diff --git a/src/java.desktop/share/legal/mesa3d.md 
b/src/java.desktop/share/legal/mesa3d.md
--- a/src/java.desktop/share/legal/mesa3d.md
+++ b/src/java.desktop/share/legal/mesa3d.md
@@ -1,10 +1,10 @@
-## Mesa 3-D Graphics Library v4.1
+## Mesa 3-D Graphics Library v5.0

  ### Mesa License
  <pre>

  Mesa 3-D graphics library
-Version:  4.1
+Version:  5.0

  Copyright (C) 1999-2002  Brian Paul   All Rights Reserved.

-phil.