NIFI-8778: update maven requirements

[ottobackwards]

Thank you for submitting a contribution to Apache NiFi.

Please provide a short description of the PR here:

Description of PR

Update the enforcer and readme for maven 3.6.0 requirement.
Tested with 3.6.0

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to ensure the following steps have been taken:

For all changes:

• Is there a JIRA ticket associated with this PR? Is it referenced
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• Have you ensured that the full suite of tests is executed via mvn -Pcontrib-check clean install at the root nifi folder?
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Note:

Please ensure that once the PR is submitted, you check GitHub Actions CI for build issues and submit an update to your PR as soon as possible.

[mattyb149]

+1 LGTM, Github Actions is happy so I'm happy :) Thanks for the improvement! Merging to main