Reduce Ubuntu 20 ARM package size

This removes unnecessary gRPC source and binaries that were bundled with
the ARM package. Prior to this change, the `grpc-1.30.2` gem was taking
220 MB of space. We can remove a number of files in the
`/opt/gitlab/embedded/lib/ruby/gems/2.7.0/gems/grpc-1.30.2` directory:

1. `/include`: 400K
2. `/src/core`: 9.3 MB
3. `/src/ruby/ext`: 196.1 MB
2. `/third_party`: 12.1 MB

These directories do not include any Ruby code, so it is safe to delete
them. `/src/ruby/ext` contains mostly C source and headers, along with
compiled objects.

Closes https://gitlab.com/gitlab-org/omnibus-gitlab/-/issues/5661

changelogs/unreleased/sh-reduce-arm-build-size.yml

+---
+title: Reduce Ubuntu 20 ARM package size
+merge_request: 4637
+author:
+type: other

config/projects/gitlab.rb

@@ -208,8 +208,12 @@ exclude 'embedded/lib/ruby/gems/*/gems/*/sample'
 exclude 'embedded/lib/ruby/gems/*/gems/*/script'
 exclude 'embedded/lib/ruby/gems/*/gems/*/t'
-# Exclude additional test files from specific gems
+# Exclude additional files from specific gems
+exclude 'embedded/lib/ruby/gems/*/gems/grpc-*/include'
+exclude 'embedded/lib/ruby/gems/*/gems/grpc-*/src/core'
+exclude 'embedded/lib/ruby/gems/*/gems/grpc-*/src/ruby/ext'
 exclude 'embedded/lib/ruby/gems/*/gems/grpc-*/src/ruby/spec'
+exclude 'embedded/lib/ruby/gems/*/gems/grpc-*/third_party'
 # Enable signing packages
 package :rpm do