doc, tls: deprecate createSecurePair

Pull Request check-list

Please make sure to review and check all of these items:

• Does make -j8 test (UNIX) or vcbuild test nosign (Windows) pass with this change (including linting)?
• Is the commit message formatted according to CONTRIBUTING.md?
• If this change fixes a bug (or a performance problem), is a regression test (or a benchmark) included?
• Is a documentation update included (if this change modifies existing APIs, or introduces new ones)?

NOTE: these things are not required to open a PR and can be done afterwards / while the PR is open.

Affected core subsystem(s)

Please provide affected core subsystem(s) (like buffer, cluster, crypto, etc)

Description of change

createSecurePair uses tls_legacy and the legacy Connection from node_crypto.cc. Deprecate them in favor of TLSSocket.

See #5924