Improve output from mariadb-secure-installation to be more honest

[ottok]

• [ ] The Jira issue number for this PR is: MDEV-______

Description

The script is likely misleading to users stating 'SUCCESS' about things it didn't do anything about, leading to false sense of security. Improve the output to be more clear about what it is doing or is not doing.

Release Notes

How can this PR be tested?

Run the script in a container with a fresh MariaDB installation.

This has been in Debian (https://salsa.debian.org/mariadb-team/mariadb-server/-/commit/785f072ac8230a57d683cf07ced225a72ed9a9c7) for 5 months without any reports of issues.

Basing the PR against the correct MariaDB version

• [x] This is a new feature or a refactoring, and the PR is based against the main branch.

PR quality check

• [x] I checked the CODING_STANDARDS.md file and my PR conforms to this where appropriate.
• [x] For any trivial modifications to the PR, I am ok with the reviewer making the changes themselves.