Update source/target to 1.8+

[pradeesh-kumar]

The source and target level specified as 1.6 in pom.xml. This restricts contributors from using the latest features of Java. Can this be changed to 1.8 or 11?

`
<plugin>

<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-compiler-plugin</artifactId>
<configuration>
    <source>1.6</source>
    <target>1.6</target>
</configuration>

</plugin>
`

[garyrussell]

This will need a bump to 2.0; we can't change the java version in a patch or minor release.

#184

[mdeinum]

Now that Spring 6 and Spring Boot 3 are in development and using Java 17 as a baseline, this might be an opportunity to start version 2.0 of spring-retry and bump the baseline to java17.

[garyrussell]

@mdeinum Yes; we have started a discussion on that internally.

[artembilan]

Has been fixed via: #286