fixing license tag in package.xml to conform to SPDX

Updating the package.xml license tags to hold valid SPDX qualifiers.
The rationale behind this is, that with
[meta-ros](https://github.com/ros/meta-ros) and
[superflore](https://github.com/ros-infrastructure/superflore)
recipes get generated for the OpenEmbedded Linux build system.
It will fetch the license tag and copy it over into the recipe. If
no valid SPDX identifiers get found, it will issue an error, and a
manual step to fix this is needed.

See the SPDX identifiers [CC0-1.0](https://spdx.org/licenses/CC0-1.0) and
[BSL-1.0](https://spdx.org/licenses/BSL-1.0.html)

Signed-off-by: Matthias Schoepfer <[email protected]>

Author: Matthias Schoepfer

tcb_span/package.xml

@@ -6,7 +6,7 @@
   <description>Implementation of C++20's std::span</description>
   <maintainer email="[email protected]">Tyler Weaver</maintainer>
 
-  <license>Boost Software License</license>
+  <license>BSL-1.0</license>
   <url>https://github.com/tcbrindle/span</url>
 
   <buildtool_depend>ament_cmake</buildtool_depend>

tl_expected/package.xml

@@ -7,7 +7,7 @@
   <maintainer email="[email protected]">Tyler Weaver</maintainer>
 
   <author>Sy Brand</author>
-  <license>Creative Commons Zero v1.0 Universal</license>
+  <license>CC0-1.0</license>
   <url>https://github.com/TartanLlama/expected</url>
 
   <buildtool_depend>ament_cmake</buildtool_depend>