Deprecate jackson.version in favour of new jackson-bom.version

The versions of Jackon's bom and the versions of Jackson are often
not the same. For example, at the time of writing, the latest bom
version is 2.9.9.20190807 but the latest version of Jackson's core
is 2.9.9 and the latest version of Jackson's databind is 2.9.9.3.

spring-boot-dependencies using a property, jackson.version, to control
the version of jackson-bom that is imported. To make it clear that the
version property controls the version of Jackson's bom and not of
Jackson itself, this commit deprecates jackson.version in favour of
a new jackson-bom.version property.

An additional benefit of this change is that it addresses a problem
when using Gradle and the dependency management plugin. Jackson's bom
uses a jackson.version property to control the version
of core Jackson modules. This names clashes with the name of the
property used by spring-boot-dependencies. Specifying a custom
jackson.version to control the version of the bom that is imported
would also have the unwanted side-effect of customizing the version of
the core Jackson modules declared in Jackson's bom.

Fixes gh-17808

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

@@ -81,7 +81,9 @@
 		<httpcore.version>4.4.11</httpcore.version>
 		<infinispan.version>9.4.15.Final</infinispan.version>
 		<influxdb-java.version>2.14</influxdb-java.version>
+		<!-- deprecated in favor of "jackson-bom.version" -->
 		<jackson.version>2.9.9.20190807</jackson.version>
+		<jackson-bom.version>${jackson.version}</jackson-bom.version>
 		<janino.version>3.0.15</janino.version>
 		<javax-activation.version>1.2.0</javax-activation.version>
 		<javax-annotation.version>1.3.2</javax-annotation.version>
@@ -621,7 +623,7 @@
 			<dependency>
 				<groupId>com.fasterxml.jackson</groupId>
 				<artifactId>jackson-bom</artifactId>
-				<version>${jackson.version}</version>
+				<version>${jackson-bom.version}</version>
 				<scope>import</scope>
 				<type>pom</type>
 			</dependency>