[YSQL] Decouple YBIsDBCatalogVersionMode() from multi-tenancy use case

[myang2021]

Jira Link: DB-7334

Description

The function YBIsDBCatalogVersionMode() was originally introduced for multi-tenancy use case. So when it returns true, it used to assume there are many databases. This is no longer true because we now also support per-database catalog version mode in single tenant clusters. We should go over the use of YBIsDBCatalogVersionMode() to fix any place where multi-tenancy use case is implied.

This was noticed when reviewing the following code:

    /*
     * YB mode uses local-tserver prefetching instead of relcache file.
     * TODO: either put this under a GUC variable or remove the old code
     * below.
     */
    if (IsYugaByteEnabled() &&
        *YBCGetGFlags()->ysql_catalog_preload_additional_tables &&
        !YBIsDBCatalogVersionMode())
        return false;

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