Add managed mimepull dependency

Add a managed dependency for `org.jvnet.mimepull:mimepull` so that
`spring-boot-starter-web-services` works with Java 11.

Although we don't directly depend on mimepull ourselves, there is an
transitive dependency from `com.sun.xml.messaging.saaj:saaj-impl:1.5.0`
which unfortunately references a version that was never published to
Maven Central.

Closes gh-14924

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

@@ -131,6 +131,7 @@
 		<lombok.version>1.18.2</lombok.version>
 		<mariadb.version>2.3.0</mariadb.version>
 		<micrometer.version>1.1.0-SNAPSHOT</micrometer.version>
+		<mimepull.version>1.9.10</mimepull.version>
 		<mockito.version>2.23.0</mockito.version>
 		<mongo-driver-reactivestreams.version>1.9.2</mongo-driver-reactivestreams.version>
 		<mongodb.version>3.8.2</mongodb.version>
@@ -2422,6 +2423,11 @@
 				<scope>import</scope>
 				<type>pom</type>
 			</dependency>
+			<dependency>
+				<groupId>org.jvnet.mimepull</groupId>
+				<artifactId>mimepull</artifactId>
+				<version>${mimepull.version}</version>
+			</dependency>
 			<dependency>
 				<groupId>org.liquibase</groupId>
 				<artifactId>liquibase-core</artifactId>