Correct the docs for last name supported keys field for the SAML SSO assertions.

What does this MR do?

It corrects the SAML SSO supported keys for last name. It incorrectly lists firstName in the last name section. This changes it to say lastName

Related issues

Closes

Author's checklist

• Apply the correct labels and milestone
• Crosslink the document from the higher-level index
• Crosslink the document from other subject-related docs
• Correctly apply the product badges and tiers
• Port the MR to EE (or backport from CE): always recommended, required when the ee-compat-check job fails

Review checklist

• Your team's review (required)
• PM's review (recommended, but not a blocker)
• Technical writer's review (required)
• Merge the EE-MR first, CE-MR afterwards