[DOCS][9.x]:Update depreciation notice for index mapping updates privileges

[charlotte-hoblik]

This PR updates the depreciation notice for 4 write-related operations on indices:

• create
• create_doc
• index
• write

Based on: elastic/docs-content#1668
8.x Version: #130916

[elasticsearchmachine]

Pinging @elastic/es-docs (Team:Docs)

[github-actions]

🔍 Preview links for changed docs

• docs/reference/elasticsearch/security-privileges.md

[charlotte-hoblik]

Hi @3kt, quick question regarding this issue:

In 8.x, we’ve included this note, that these privileges no longer grant index mapping updates, and that this behavior will be removed in a future major release.

Now that we’re in 9.x, should we:
1. Keep this note (if the functionality is still partially supported but deprecated), or
2. Remove or revise it in 9.x (if mapping updates via these privileges are already fully removed)?

Just want to make sure we’re accurately reflecting the behavior across versions. Thanks!