Use the SPDX identifier Apache-2.0 in package.xml license tags (ros2#604)

The <license> tags spelled the license as prose ("Apache License 2.0",
"Apache 2.0"). Use the SPDX identifier instead so the declaration can be
matched mechanically. The license itself does not change.

Assisted-by: Claude Code:claude-opus-5 [git, gh]

Signed-off-by: Michael Carroll <mjcarroll.oss@gmail.com>

Author: mjcarroll

package.xml

@@ -9,7 +9,7 @@
   <maintainer email="clalancette@openrobotics.org">Chris Lalancette</maintainer>
   <maintainer email="william@openrobotics.org">William Woodall</maintainer>
 
-  <license>Apache License 2.0</license>
+  <license>Apache-2.0</license>
 
   <author email="karsten@osrfoundation.org">Karsten Knese</author>