Update license headers to explicitly say GPLv2+
diff --git a/HeaderOptionsPanel.c b/HeaderOptionsPanel.c
index 701f6d6..d8148df 100644
--- a/HeaderOptionsPanel.c
+++ b/HeaderOptionsPanel.c
@@ -1,7 +1,7 @@
 /*
 htop - HeaderOptionsPanel.c
 (C) 2021 htop dev team
-Released under the GNU GPLv2, see the COPYING file
+Released under the GNU GPLv2+, see the COPYING file
 in the source distribution for its full text.
 */