Migrate to Maven 3.7 and remove mvnw

[fulldecent]

Blockers:

• Maven 3.7 release // announced at http://maven.apache.org/download.cgi

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mvnw is moving into the upstream project and so the standalone project is deprecated.

Source: takari/maven-wrapper#139

So we can plan to upgrade to Maven 3.7 and then remove mvnw here.

[satran004]

Thanks @fulldecent for highlighting this.
We can start planning for mvn 3.7. So that when it's out, we are compatible.
But we may need to support the older version for sometime, as IDE may take sometime to bundle the new maven version.