refactor: use system admin word correctly

[kentakozuka]

This pull request primarily focuses on renaming the error ErrAdminAccountNotFound to ErrSystemAdminAccountNotFound and updating the associated SQL query in the pkg/account package. The changes ensure that the correct error and SQL query are used when retrieving a system admin account.

Here are the key changes:

• pkg/account/storage/v2/admin_account.go: Renamed the error ErrAdminAccountNotFound to ErrSystemAdminAccountNotFound and updated the SQL query from selectAdminAccountV2SQL to selectSystemAdminAccountV2SQL. This change affects the GetSystemAdminAccountV2 method where the new error is returned when the system admin account is not found. [1] [2]

• pkg/account/api/admin_account.go: Updated the getSystemAdminAccountV2 method in the AccountService to check for the new error ErrSystemAdminAccountNotFound instead of the old error.

• pkg/account/api/admin_account_test.go and pkg/account/storage/v2/admin_account_test.go: Updated the test cases to expect the new error ErrSystemAdminAccountNotFound instead of the old error. [1] [2]

[cre8ivejp]

Thanks!
It would be better to give another commit message because you already merged one using the same message.

E.g.

refactor: renaming error messages in the account for easier understanding
or
refactor: adding prefix `system` for easier understanding