docs: update security policy and upgrade guide links to canonical URLs

sai krishna chowdaryrequested to merge
kittusontineni/terraform-provider-gitlab:docs-security-policy-canonical-links into main

What does this MR do?

Updates documentation and handbook links across docs/data-sources/security_policy_document.md and docs/guides/version-15.7-upgrade.md to canonical URLs, removing deprecated ee/ path prefixes and pointing to handbook.gitlab.com.

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