fix(owlbot-postprocessor): remove default json gemspec to clear CVE

[torreypayne]

Clears the json-2.18.0.gemspec file from the system Ruby default gems inside the image, mitigating the scanner alert.

Fixes b/548806296

Validation & Repro Steps

# 1. Confirm the published (vulnerable) image still triggers the alert by checking for the default json gemspec
docker pull gcr.io/cloud-devrel-public-resources/owlbot-ruby:latest
docker run --rm --entrypoint /bin/sh gcr.io/cloud-devrel-public-resources/owlbot-ruby:latest -c 'ls -l /usr/local/lib/ruby/gems/*/specifications/default/json*.gemspec'
# -> Expected output: -rw-r--r-- ... /usr/local/lib/ruby/gems/4.0.0/specifications/default/json-2.18.0.gemspec

# 2. Navigate to the owlbot-postprocessor directory in the repository
cd owlbot-postprocessor

# 3. Build the patched owlbot-ruby image locally
docker build -t owlbot-ruby:patched .

# 4. Confirm the vulnerable system default json gem has been successfully scrubbed
docker run --rm --entrypoint /bin/sh owlbot-ruby:patched -c 'ls -l /usr/local/lib/ruby/gems/*/specifications/default/json*.gemspec'
# -> Expected output: "ls: cannot access ... No such file or directory"

# 5. Confirm the securely patched json:2.19.2 gem remains installed and active
docker run --rm --entrypoint /bin/sh owlbot-ruby:patched -c 'ls -l /usr/local/bundle/specifications/json*.gemspec'
# -> Expected output: -rw-r--r-- ... /usr/local/bundle/specifications/json-2.19.2.gemspec

[aandreassa]

LGTM, minor nits.

[aandreassa]

Please make sure you merge the new image SHA once a PR pops up on google-cloud-ruby later.

[aandreassa]

Perfect, ty!