Update version compatibility upgrade banner to clarify upgrade path r… (#34204)

…estrictions

- Specifically disallowing upgrades from 25.2 to **26.X.Y** without
first upgrading to 25.2.15

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I have pinged the relevant PM to schedule a changelog post.

Author: SangJunBak

doc/user/config.toml

@@ -7,7 +7,7 @@ disableKinds = ['taxonomy']
 
 [params]
 repo = "//github.com/MaterializeInc/materialize"
-bannerMessage = "**New: Self-Managed v26.0!** New features include license keys, SCRAM-SHA support, and more! For details on v26.0 features, see [Release Notes](/releases/).<br>**Note**: To upgrade from v25.2 to v26.0, you must [upgrade first to v25.2.15](https://materialize.com/docs/self-managed/v25.2/release-notes/#v25215)."
+bannerMessage = "**New: Self-Managed v26.0!** New features include license keys, SCRAM-SHA support, and more! For details on v26.0 features, see [Release Notes](/releases/).<br>**Note**: To upgrade from v25.2 to v26.0, you must [upgrade first to v25.2.15+](https://materialize.com/docs/self-managed/v25.2/release-notes/#v25215)."
 bannerLink = ""
 
 [frontmatter]

doc/user/shared-content/self-managed/version-compatibility-upgrade-banner.md

@@ -3,6 +3,6 @@
 When performing major version upgrades, you can upgrade only one major version
 at a time. For example, upgrades from **v26**.1.0 to **v27**.2.0 is permitted
 but **v26**.1.0 to **v28**.0.0 is not. Skipping major versions or downgrading is
-not supported.
+not supported. To upgrade from v25.2 to v26.0, you must [upgrade first to v25.2.15+](https://materialize.com/docs/self-managed/v25.2/release-notes/#v25215).
 
 {{</ important >}}