Extract spring-doc-resources URL property

Update the build to use a shared spring-doc-resources URL
rather than repeating it multiple times.

See gh-18293

spring-boot-project/spring-boot-actuator-autoconfigure/pom.xml

@@ -648,7 +648,7 @@
 									<goal>wget</goal>
 								</goals>
 								<configuration>
-									<url>https://repo.spring.io/snapshot/io/spring/docresources/spring-doc-resources/${spring-doc-resources.version}/spring-doc-resources-${spring-doc-resources.version}.zip</url>
+									<url>${spring-doc-resources.url}</url>
 									<unpack>true</unpack>
 									<outputDirectory>${refdocs.build.directory}</outputDirectory>
 								</configuration>

spring-boot-project/spring-boot-docs/pom.xml

@@ -1193,7 +1193,7 @@
 									<goal>wget</goal>
 								</goals>
 								<configuration>
-									<url>https://repo.spring.io/snapshot/io/spring/docresources/spring-doc-resources/${spring-doc-resources.version}/spring-doc-resources-${spring-doc-resources.version}.zip</url>
+									<url>${spring-doc-resources.url}</url>
 									<unpack>true</unpack>
 									<outputDirectory>${refdocs.build.directory}</outputDirectory>
 								</configuration>

spring-boot-project/spring-boot-parent/pom.xml

@@ -29,6 +29,7 @@
 		<testcontainers.version>1.12.0</testcontainers.version>
 		<testng.version>6.14.3</testng.version>
 		<spring-doc-resources.version>0.1.3.BUILD-SNAPSHOT</spring-doc-resources.version>
+		<spring-doc-resources.url>https://repo.spring.io/snapshot/io/spring/docresources/spring-doc-resources/${spring-doc-resources.version}/spring-doc-resources-${spring-doc-resources.version}.zip</spring-doc-resources.url>
 	</properties>
 	<scm>
 		<url>https://github.com/spring-projects/spring-boot</url>

spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/pom.xml

@@ -50,7 +50,7 @@
 							<goal>wget</goal>
 						</goals>
 						<configuration>
-							<url>https://repo.spring.io/release/io/spring/docresources/spring-doc-resources/${spring-doc-resources.version}/spring-doc-resources-${spring-doc-resources.version}.zip</url>
+							<url>${spring-doc-resources.url}</url>
 							<unpack>true</unpack>
 							<outputDirectory>${refdocs.build.directory}/asciidoc</outputDirectory>
 						</configuration>