licenses: Add Elastic License 2.0 (Elasticv2)

[marclop]

What does this PR do?

Adds the Elastic License 2.0 as Elasticv2 in the licenses package.
Updates the go-licenser to 0.4.0, since it adds support for the
Elasticv2 license.

Note that it does not modify the Beats license since that's out of
the scope of this PR.

Why is it important?

It unblocks elastic/apm-server#6105 and I believe may unblock some
work that's pending for Beats to switch to ELv2.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• [ ] I have added tests that prove my fix is effective or that my feature works
• [ ] I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

Author's Checklist

• Dev-tools.
• Licenses package.

Related issues

Unblocks elastic/apm-server#6105

[elasticmachine]

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Build stats

• Start Time: 2021-11-04T04:06:27.103+0000

• Duration: 202 min 52 sec

• Commit: a8f1c14

Test stats 🧪

Test	Results
Failed	0
Passed	54119
Skipped	5355
Total	59474

💚 Flaky test report

Tests succeeded.

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[mergify]

This pull request is now in conflicts. Could you fix it? 🙏
To fixup this pull request, you can check out it locally. See documentation: https://help.github.com/articles/checking-out-pull-requests-locally/

git fetch upstream
git checkout -b f/add-elastic-license-2.0 upstream/f/add-elastic-license-2.0
git merge upstream/master
git push upstream f/add-elastic-license-2.0