Use proper SPDX license identifier (#9441)

The previously used term "3-Clause BSD License" is not properly
standarized. A common standard is SPDX, therefore "3-Clause BSD License"
is substituted with "BSD-3-Clause" which is a SPDX identifier.

`grep -rl "3-Clause BSD License" | xargs -n1 sed -i "s/3-Clause BSD
License/BSD-3-Clause/g"`
diff --git a/ruby/pom.xml b/ruby/pom.xml
index 255d840..d4a8c10 100644
--- a/ruby/pom.xml
+++ b/ruby/pom.xml
@@ -19,7 +19,7 @@
     <url>https://developers.google.com/protocol-buffers/</url>
     <licenses>
       <license>
-        <name>3-Clause BSD License</name>
+        <name>BSD-3-Clause</name>
         <url>https://opensource.org/licenses/BSD-3-Clause</url>
         <distribution>repo</distribution>
       </license>