docs: mark CREATE TABLE ... FROM SOURCE as Public Preview

[claude]

Requested by Pranshu Maheshwari · Slack thread

Motivation

The new "table from source" / source-versioning syntax has officially graduated
to Public Preview. The v26.25 release note states: "Source versioning has
graduated from private preview to public preview, and is now available by
default across all environments." The user docs should no longer show a
Private Preview banner for this feature.

Description

Most of the docs for this feature were already Public Preview:

• doc/user/content/sql/create-table.md (the CREATE TABLE ... FROM SOURCE
  reference) — already uses {{< public-preview />}} on all source-table tabs.
  No Private Preview banners remained here — the reference page was already fixed.
• doc/user/content/ingest-data/{postgres,mysql,sql-server}/source-versioning.md
  — already Public Preview.
• doc/user/content/sql/create-source/mysql-v2.md (MySQL New Syntax) — already
  Public Preview.

The only stragglers still carrying a Private Preview banner were the two
sibling "New Syntax" CREATE SOURCE reference pages — the source half of the
same table-from-source workflow:

• doc/user/content/sql/create-source/postgres-v2.md (line 12) — flipped {{< private-preview />}} → {{< public-preview />}}
• doc/user/content/sql/create-source/sql-server-v2.md (line 13) — flipped {{< private-preview />}} → {{< public-preview />}}

These were flipped for consistency with the sibling mysql-v2.md page and with
the v26.25 release note.

Scope note for the reviewer: The banners flipped here live on CREATE SOURCE
(New Syntax) pages rather than literally on CREATE TABLE ... FROM SOURCE. They
belong to the same "source versioning" feature that graduated, so flipping them
completes the rollout. If you consider these out of scope for this change, they
can be dropped — the literal CREATE TABLE ... FROM SOURCE reference required no
changes and was already Public Preview.

The RETAIN HISTORY "Private preview" notes in create_table_user_populated.yml
and alter_table.yml, and unrelated Private Preview banners elsewhere, were
intentionally left untouched.

Verification

• rg -i "private.preview" doc/user/content/sql/create-table.md returns no
  matches (only public-preview), confirming the CREATE TABLE FROM SOURCE
  reference is fully Public Preview.
• After the change, no create-source/*-v2.md page carries a Private Preview
  banner; all three (PostgreSQL, MySQL, SQL Server) are Public Preview.
• Change is docs-only (two Hugo shortcode swaps); no code paths affected.

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