feat(NODE-6158): add signature to github releases

[aditi-khare-mongoDB]

Description

Sign releases in legacy driver.

What is changing?

Automate release signing with a detached signature and verification instructions in the README.
Link to an example of release signatures working with v2 (if you check the artifact link at the end of the compress_sign step, you can see the signature)

Is there new documentation needed for these changes?

No

What is the motivation for this change?

SSDLC Compliance

Release Highlight

Add Signature to Github Releases

The Github release for mongodb-legacy now contains a detached signature file for the NPM package (named
mongodb-legacy-X.Y.Z.tgz.sig), on every major and patch release. To verify the signature, follow the instructions in the 'Release Integrity' section of the README.md file.

Double check the following

• Ran npm run check:lint script
• Self-review completed using the steps outlined here
• PR title follows the correct format: type(NODE-xxxx)[!]: description
  • Example: feat(NODE-1234)!: rewriting everything in coffeescript
• Changes are covered by tests
• New TODOs have a related JIRA ticket

[W-A-James]

LGTM, just one question

[W-A-James]

Also just need to update the PR description since we did end up migrating to the v2 action