feat(stackdriver)!: tombstone stackdriver gem

[torreypayne]

feat(stackdriver)!: Replace the stackdriver gem with a tombstone

The Stackdriver branding was discontinued long ago, and the team does not have the bandwidth to maintain the "batteries included" SDK suite originally intended for Stackdriver.

This PR replaces the stackdriver gem with a minimal tombstone in preparation for a semver-major release (1.0.0).

Details of Changes

• Runs the tombstone-library Toys task to prepare the gem for retirement.
• Bumps the major version to 1.0.0 to signal a breaking change (removal of all functionality).
• Removes all runtime dependencies from stackdriver.gemspec (so it no longer pulls in google-cloud-logging, google-cloud-error_reporting, or google-cloud-trace).
• Adds a post-install message to the gemspec and updates the README.md to notify users that the gem is obsolete.
• Deletes all functional code, leaving only lib/stackdriver/version.rb, README.md, LICENSE.md, and .yardopts.

Dependency Analysis

• Checked all active gems inside google-cloud-ruby and verified that none depend on the stackdriver gem.
• The appengine gem in the archived GoogleCloudPlatform/appengine-ruby repository relies on stackdriver (~> 0.21). Releasing the tombstone as a major version 1.0.0 ensures that older applications using the appengine gem will resolve to the last compatible release (0.24.0) rather than the zero-dependency tombstone.

Fixes: b/407601625
TAG=agy
CONV=06672b79-ff5c-48a1-b60d-3b5bbee17693

[aandreassa]

LGTM. We can release on Monday! Please file a bug to fix the workflow if possible.