Bump JDK version to 1.8

Inspired by redis#1334 where this went real easy :).

Would have made redis#2355 shorter.

Free public updates for JDK 7 ended in 2015:
<https://en.wikipedia.org/wiki/Java_version_history>

For JDK 8, free public support is available from non-Orace vendors until
at least 2026 according to the same table.

And JDK 8 is what Jedis is being tested on anyway:
<https://github.com/redis/jedis/blob/ac0969315655180c09b8139c16bded09c068d498/.circleci/config.yml#L67-L74>

pom.xml

@@ -129,8 +129,8 @@
 				<artifactId>maven-compiler-plugin</artifactId>
 				<version>3.8.1</version>
 				<configuration>
-					<source>1.7</source>
-					<target>1.7</target>
+					<source>1.8</source>
+					<target>1.8</target>
 				</configuration>
 			</plugin>
 			<plugin>