chore: Change signature public key name

[cveticm]

Proposed changes

Changes the name of public key to signature.asc as part of work for CLOUDP-297009. This unifies the pub key name to ensure that, if any bug arises that causes another pub key to be included in the release, AtlasCLI will collect the correct key.

Note: Failure in E2E unrelated to changes in this PR.

Jira ticket: CLOUDP-#

Checklist

• I have signed the MongoDB CLA
• I have added tests that prove my fix is effective or that my feature works
• I have added any necessary documentation in the document requirements section listed in CONTRIBUTING.md (if appropriate)
• I have addressed the @mongodb/docs-cloud-team comments (if appropriate)
• I have run make fmt and formatted my code

Further comments