update gradle wrapper to 6.7.1

[akhomchenko]

Thank you a lot for library! I find it useful and interesting to study. I want to contribute some features but local build fails for me so I've decided to start with fix for that.

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Change includes:

• https instead of http for spring repo as http endpoint is deprecated
• move properties after plugins as otherwise some properties are not resolved
• replace '<<' with doLast as '<<' is deprecated since Gradle 5.0
• remove osgi plugin as it is deprecated and I am not sure it does something useful
• remove wrapper task as updates can be done via built-in gradle wrapper task
• replace broken javadoc links with working