Document security disclosure responsibilities in MAINTAINERS.md

[evan2645]

SPIRE maintainers receive security disclosures, however the responsibility placed on them in the handling of these disclosures is not documented in MAINTAINERS.md.

Document maintainer responsibility as it relates to security response procedures, including confidentiality and response time requirements.

[amoore877]

This seems straightforward to do but has been open a while; is the missing piece a discussion on what the complete list of procedures and requirements should be?

[github-actions[bot]]

This issue is stale because it has been open for 365 days with no activity.

[github-actions[bot]]

This issue was closed because it has been inactive for 30 days since being marked as stale.