Update NEWS post GDB 10 branch creation.

gdb/ChangeLog:

	* NEWS: Create a new section for the next release branch.
	Rename the section of the current branch, now that it has
	been cut.

gdb/ChangeLog

@@ -1,3 +1,9 @@
+2020-09-13  Joel Brobecker  <brobecker@adacore.com>
+
+	* NEWS: Create a new section for the next release branch.
+	Rename the section of the current branch, now that it has
+	been cut.
+
 2020-09-13  Joel Brobecker  <brobecker@adacore.com>
 
 	GDB 10 branch created (8087c3fa8b5d695e3e29e69d70d0b35ec902ac59):

gdb/NEWS

@@ -1,7 +1,9 @@
 		What has changed in GDB?
 	     (Organized release by release)
 
-*** Changes since GDB 9
+*** Changes since GDB 10
+
+*** Changes in GDB 10
 
 * There are new feature names for ARC targets: "org.gnu.gdb.arc.core"
   and "org.gnu.gdb.arc.aux".  The old names are still supported but