logictest: fix user logictest for provisioning gating validation

The current logictest validates the version gating logic for provisioning but
checks for skipif version instead of onlyif version config which was pointed out
in the discussion
#149405 (comment) and
needs to change. Additionally, we need to add a comment for version gating logic
as both are failure scenarios owing to "cert-password" currently being a non
support provisioning auth method as in the discussion
#149405 (comment).

fixes #147599
Epic CRDB-21590

Release note: None

Author: souravcrl

pkg/sql/logictest/testdata/logic_test/user

@@ -227,14 +227,13 @@ user root
 statement ok
 set cluster setting security.provisioning.ldap.enabled = true;
 
-skipif config local 3node-tenant
+onlyif config local-mixed-25.2
 statement ok
 DROP user testuser
 
-skipif config local 3node-tenant
+onlyif config local-mixed-25.2
 user testuser nodeidx=0 newsession
 
-skipif config local 3node-tenant
 statement error pq: password authentication failed for user testuser
 SHOW session_user
 
@@ -254,6 +253,11 @@ DROP user testuser
 skipif config local-mixed-25.2
 user testuser nodeidx=0 newsession
 
+# The logictest suite currently doesn't work for external authentication methods
+# as the register auth method for "ldap" is invoked outside of the sql package.
+# For the purpose of validating the release flags for provisioning, test only
+# auth method "cert-password" has been enabled which hasn't been provided with a
+# provisioner(handler for provisioning) which is the obtained error here.
 skipif config local-mixed-25.2
 statement error pq: user identity unknown for identity provider
 SHOW session_user