Update elastic-package version for constant_keyword value

[andrewkroh]

What does this PR do?

Update to elastic/elastic-package@2278b9e. This adds validation that if a document contains a constant_keyword field that its value must match the declared value from the fields definition (if declared).

Related issues

• Relates Add support for specifying a constant_keyword value package-spec#194
• Relates Add support for constant_keyword "value" in field definition elastic-package#386

[elasticmachine]

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• Build Cause: Branch indexing

• Start Time: 2021-06-28T22:28:23.328+0000

• Duration: 67 min 32 sec

• Commit: 16f8bd1

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[mtojek]

I will merge the PR as the CI is happy.