Reduce gem package size

[pocke]

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Overview

This pull request reduces gem package size.

Currently newrelic_rpm gem package is 4.5MB after unzip. We can confirm the size with du command.
For example

$ cd path/to/installed/dir/lib/ruby/gems/2.7.0/gems
$ du newrelic_rpm-6.13.1/ --summarize -h
4.5M    newrelic_rpm-6.13.1/

And .github directory occupies half of the size.

# in the same directory
$ du newrelic_rpm-6.13.1/* --summarize -b | sort -n | tail
7933    newrelic_rpm-6.13.1/README.md
9317    newrelic_rpm-6.13.1/config.dot
11571   newrelic_rpm-6.13.1/LICENSE
13169   newrelic_rpm-6.13.1/bin/
17626   newrelic_rpm-6.13.1/THIRD_PARTY_NOTICES.md
33302   newrelic_rpm-6.13.1/test/
69358   newrelic_rpm-6.13.1/cert/
177295  newrelic_rpm-6.13.1/CHANGELOG.md
1285344 newrelic_rpm-6.13.1/lib/
2178011 newrelic_rpm-6.13.1/.github/

But .github directory is not necessary for the package users. So this pull request removes the .github directory from the gem package.
It reduces 2.2MB package size.

# In the same directory
$ du newrelic_rpm-6.13.1/ --summarize -h
2.3M    newrelic_rpm-6.13.1/

Related Github Issue

nothing

Testing

I've confirmed the existing test suites are all green.

[CLAassistant]

All committers have signed the CLA.

[mwlang]

LGTM -- removes .github from the manifest, which is where the Github Actions workflow files live and have no bearing on the packaged gem file.