Don't sync spec folder from Rails' vendored gems

What does this MR do?

Excluding these files should save ~1.7MiB:

root@16.8.1-ee:/opt/gitlab/embedded/service/gitlab-rails# du -shc
gems/*.yml gems/config gems/README.md gems/*/spec vendor/gems/*/spec
vendor/gems/README.md
4.0K    gems/gem-pg.gitlab-ci.yml
4.0K    gems/gem.gitlab-ci.yml
8.0K    gems/config
4.0K    gems/README.md
84K     gems/activerecord-gitlab/spec
52K     gems/click_house-client/spec
28K     gems/csv_builder/spec
32K     gems/gitlab-backup-cli/spec
8.0K    gems/gitlab-database-load_balancing/spec
20K     gems/gitlab-database-lock_retries/spec
52K     gems/gitlab-housekeeper/spec
120K    gems/gitlab-http/spec
20K     gems/gitlab-rspec/spec
52K     gems/gitlab-rspec_flaky/spec
28K     gems/gitlab-safe_request_store/spec
200K    gems/gitlab-schema-validation/spec
32K     gems/gitlab-secret_detection/spec
76K     gems/gitlab-utils/spec
432K    gems/ipynbdiff/spec
152K    vendor/gems/cloud_profiler_agent/spec
40K     vendor/gems/devise-pbkdf2-encryptable/spec
28K     vendor/gems/mail-smtp_pool/spec
52K     vendor/gems/microsoft_graph_mailer/spec
28K     vendor/gems/omniauth-azure-oauth2/spec
20K     vendor/gems/omniauth-gitlab/spec
24K     vendor/gems/omniauth-salesforce/spec
48K     vendor/gems/omniauth_crowd/spec
36K     vendor/gems/sidekiq-reliable-fetch/spec
4.0K    vendor/gems/README.md
1.7M    total

Related issues

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion.

Required

• MR title and description are up to date, accurate, and descriptive.
• MR targeting the appropriate branch.
• Latest Merge Result pipeline is green.
• When ready for review, MR is labeled "~workflow::ready for review" per the Distribution MR workflow.

For GitLab team members

If you don't have access to this, the reviewer should trigger these jobs for you during the review process.

• The manual Trigger:ee-package jobs have a green pipeline running against latest commit. (https://gitlab.com/gitlab-org/omnibus-gitlab/-/pipelines/1156758904)
• If config/software or config/patches directories are changed, make sure the build-package-on-all-os job within the Trigger:ee-package downstream pipeline succeeded.
• If you are changing anything SSL related, then the Trigger:package:fips manual job within the Trigger:ee-package downstream pipeline must succeed.
• If CI configuration is changed, the branch must be pushed to dev.gitlab.org to confirm regular branch builds aren't broken.

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes.
• Documentation created/updated.
• Tests added.
• Integration tests added to GitLab QA.
• Equivalent MR/issue for the GitLab Chart opened.
• Validate potential values for new configuration settings. Formats such as integer 10, duration 10s, URI scheme://user:passwd@host:port may require quotation or other special handling when rendered in a template and written to a configuration file.