Start building against Spring Framework 5 snapshot

This commit enables compatibility build against Spring Framework 5.

The Velocity and Guava support that are deprecated in the 1.x line have
been removed and few other classes contain minor change to comply to non
backward compatible changes in Spring Framework 5.

This commit also switches the required java version to 8.

Closes gh-6977

spring-boot-samples/spring-boot-sample-cache/README.adoc

@@ -10,7 +10,6 @@ abstraction is supported by many caching libraries, including:
 * `Couchbase`
 * `Redis`
 * `Caffeine`
-* `Guava`
 * Simple provider based on `ConcurrentHashMap`
 * Generic provider based on `org.springframework.Cache` bean definition(s)
 
@@ -99,11 +98,3 @@ a redis instance with the default settings is expected on your local box).
 Simply add the `com.github.ben-manes.caffeine:caffeine` dependency to enable support
 for Caffeine. You can customize how caches are created in different ways, see
 `application.properties` for an example and the documentation for more details.
-
-
-
-=== Guava
-Spring Boot does not provide any dependency management for _Guava_ so you'll have to add
-the `com.google.guava:guava` dependency with a version. You can customize how caches are
-created in different ways, see `application.properties` for an example and the
-documentation for more details.

spring-boot-samples/spring-boot-sample-cache/pom.xml

@@ -96,13 +96,6 @@
 			<artifactId>caffeine</artifactId>
 		</dependency>
 		-->
-		<!--
-		<dependency>
-			<groupId>com.google.guava</groupId>
-			<artifactId>guava</artifactId>
-			<version>18.0</version>
-		</dependency>
-		-->
 		<dependency>
 			<groupId>org.springframework.boot</groupId>
 			<artifactId>spring-boot-starter-test</artifactId>

spring-boot-samples/spring-boot-sample-cache/src/main/resources/application.properties

@@ -14,9 +14,3 @@
 # Caffeine configuration
 #
 #spring.cache.caffeine.spec=maximumSize=200,expireAfterAccess=600s
-
-
-#
-# Guava configuration
-#
-#spring.cache.guava.spec=maximumSize=200,expireAfterAccess=600s