Protect GitHub tutorial: correct the credential request and username mapping

[J-Hunter-Hawke]

This PR corrects the tutorial Protect GitHub with SSH Certificates. It changes one file: tutorials/protect-github-ssh.mdx.

• Step 1, credential request. The request now uses Create Endpoint Configuration (POST /api/endpoint-configurations) with kind: account. This is the request that creates an SSH user credential. It matches the request in the GitHub SSH guide that we already share with customers. The field list below the request uses the new field names.
• Step 1, authority. This PR removes the paragraph that tells the reader to copy the Authority ID. It also removes the authorityID field from the request. With no authority in the request, Smallstep issues the certificates from the SSH authority of the team. Smallstep SSH uses the same authority. Thus the credential works with Smallstep SSH. In Step 2, the reader already copies the user CA key of that authority.
• GitHub username. The page said in five places that the username is a certificate principal. The places are the diagram, the introduction, the field list, the end of Step 1, and Step 2. GitHub identifies the user from the login@github.com extension. Smallstep puts the githubUsername attribute in that extension. The text for SSH.Principals now says what it holds: POSIX usernames and SSH group principals.
• Commit signing. The user.signingkey command now has the prefix key::. Without the prefix, Git reads the value as a file path, unless the value starts with ssh-. The default key of the agent is ECDSA P-256 (ecdsa-sha2-nistp256 ...).
• Verify and troubleshoot. This PR removes the line that tells the reader to confirm an issued SSH certificate in the dashboard. The ssh-add -l check above that line is sufficient.
• Date. updated_at is now October 13, 2026.

For review

• The API reference marks Create Endpoint Configuration as deprecated. PRO-606 gives the background and the open question about which request this page shows.
• A test used git 2.56.0 and an ECDSA P-256 key. The value without the prefix failed. The value with the prefix key:: signed the commit.
• A review compared the fields and the values of the request with the 2025-01-01 API schema.

Related links/other PRs/issues:

• Refs PRO-606
• GitHub: About SSH certificateauthorities
• Git: user.signingKey in git-config