Remove server version upgrade (#14457) (#14503)

tidb-configuration-file.md

@@ -122,6 +122,10 @@ 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

@@ -22,7 +22,8 @@ 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 v7.1.0 or a later v7.1.x version is not supported. You need to upgrade your cluster first to v4.0 and then to the target TiDB version.
+> - If your cluster to be upgraded is v3.1 or an earlier version (v3.0 or v2.1), the direct upgrade to v7.1.0 or a later v7.1.x version is not supported. You need to upgrade your cluster first to v4.0 and then to the target TiDB version.
+> - 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