Reword NameID format to be recommended

What does this MR do?

If the NameID format of the SAML response is not persistent, a warning is thrown. The current wording along with the text and form field outline being red suggests that this is an error (and must be persistent) even though our documentation says it's "recommended" and the feature works even when it's not persistent.

Screenshots or Screencasts (strongly suggested)

Before:

After:

How to setup and validate locally (strongly suggested)

1. Turn on Group SAML.
2. Add a SAML app to the SSO Settings on a group.
3. When configuring the SAML app, ensure to set the NameID format to something aside from "Persistent".
4. Click on the "Verify SAML Configuration" button.

Does this MR meet the acceptance criteria?

Conformity

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• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
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• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Security

Does this MR contain changes to processing or storing of credentials or tokens, authorization and authentication methods or other items described in the security review guidelines? If not, then delete this Security section.

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team