Update tags for native realm authentication

[yetanothertw]

According to the Compare Elastic Cloud Hosted and Serverless page that compares various capabilities, the native realm authentication capability is not available in Serverless.

Related to #1198

[github-actions]

🔍 Preview links for changed docs

• deploy-manage/users-roles/cluster-or-deployment-auth/native.md

[shainaraskas]

careful with this one - none of the other cluster or deployment user auth stuff works for serverless either (EVERYTHING under this section, MOST of what is under this section (exceptions described below)). prob should try to tag all of these pages consistently.

side note: auth is a picky thing that is VERY deployment specific. I am ok with using stack here because we need to do other stuff to fix the all deployment types == stack issue but that's why all of these tags were here in the first place

one thing I also noticed while spot checking: we have controlling access at the document and field level tagged for serverless, but the intro page for user roles doesn't have a serverless tag. these pages should be in conversation with each other: the page at the top of the pile should always be an overview that has a superset of all of the tags of the child pages. maybe something to worry about with your other PR that I haven't looked at

this issue will also be closed when you merge this: https://github.com/elastic/docs-content-internal/issues/384

[shainaraskas]

see my other comment - if we want to change these tags in this section, we should do so holistically