[0.8.0] Drop support for Confluence 5.8 and older

[Marek Parfianowicz]

As per https://confluence.atlassian.com/support/atlassian-support-end-of-life-policy-201851003.html the oldest version supported is Confluence 5.9. Update pom.xml to run against this version instead of 5.3.

[Marek Parfianowicz]

There's no 5.9.0 JAR in https://packages.atlassian.com/maven-public-legacy/com/atlassian/confluence/confluence

There are 5.9.x JARs in https://maven.atlassian.com/content/groups/public/com/atlassian/confluence/confluence

Built successfully with 5.9.14.