Use proper SPDX license identifier (protocolbuffers#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"`

Protobuf-C++.podspec

@@ -3,7 +3,7 @@ Pod::Spec.new do |s|
  s.version = '3.19.3'
  s.summary = 'Protocol Buffers v3 runtime library for C++.'
  s.homepage = 'https://github.com/google/protobuf'
- s.license = '3-Clause BSD License'
+ s.license = 'BSD-3-Clause'
  s.authors = { 'The Protocol Buffers contributors' => '[email protected]' }
  s.cocoapods_version = '>= 1.0'
 

Protobuf.podspec

@@ -8,7 +8,7 @@ Pod::Spec.new do |s|
  s.version = '3.19.3'
  s.summary = 'Protocol Buffers v.3 runtime library for Objective-C.'
  s.homepage = 'https://github.com/protocolbuffers/protobuf'
- s.license = '3-Clause BSD License'
+ s.license = 'BSD-3-Clause'
  s.authors = { 'The Protocol Buffers contributors' => '[email protected]' }
  s.cocoapods_version = '>= 1.0'
 

java/bom/pom.xml

@@ -29,7 +29,7 @@
 
  <licenses>
  <license>
- <name>3-Clause BSD License</name>
+ <name>BSD-3-Clause</name>
  <url>https://opensource.org/licenses/BSD-3-Clause</url>
  </license>
  </licenses>

java/pom.xml

@@ -41,7 +41,7 @@
 
  <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>