Remove server version upgrade (#14457)

tidb-configuration-file.md

@@ -124,6 +124,10 @@ The TiDB configuration file supports more options than command-line parameters.
 + Default value: ""
 + By default, the format of the TiDB version string is `5.7.${mysql_latest_minor_version}-TiDB-${tidb_version}`.
 
+> **Note:**
+>
+> TiDB nodes use the value of `server-version` to verify the current TiDB version. Therefore, to avoid unexpected behaviors, before upgrading the TiDB cluster, you need to set the value of `server-version` to empty or the real version of the current TiDB cluster.
+
 ### `repair-mode`
 
 - Determines whether to enable the untrusted repair mode. When the `repair-mode` is set to `true`, bad tables in the `repair-table-list` cannot be loaded.

upgrade-tidb-using-tiup.md

@@ -23,7 +23,8 @@ This document is targeted for the following upgrade paths:
 
 > **Note:**
 >
-> If your cluster to be upgraded is v3.1 or an earlier version (v3.0 or v2.1), the direct upgrade to v7.2.0 is not supported. You need to upgrade your cluster first to v4.0 and then to v7.2.0.
+> - If your cluster to be upgraded is v3.1 or an earlier version (v3.0 or v2.1), the direct upgrade to v7.2.0 is not supported. You need to upgrade your cluster first to v4.0 and then to v7.2.0.
+> - TiDB nodes use the value of the [`server-version`](/tidb-configuration-file.md#server-version) configuration item to verify the current TiDB version. Therefore, to avoid unexpected behaviors, before upgrading the TiDB cluster, you need to set the value of `server-version` to empty or the real version of the current TiDB cluster.
 
 ## Upgrade caveat