What does this PR do?

Fixes Debian package maintainer script ownership/user handling for salt-master and salt-minion so upgrades/reinstalls don’t reset custom unprivileged users back to salt.
Specifically, the preinst/postinst scripts now:
preserve existing debconf user selections,
fall back to user: from /etc/salt/master or /etc/salt/minion when appropriate,
mark debconf entries as seen to avoid unintended resets,
use the resolved user’s primary group for ownership operations.

What issues does this PR fix or reference?

Fixes #68577

Previous Behavior

On Debian package upgrades/reinstalls/reconfigure, ownership on salt directories/files could be reset to default salt/root paths because maintainer scripts overwrote user selection when pidfiles were absent and chowned with default assumptions.

New Behavior

Debian maintainer scripts now preserve configured service users across upgrade/reinstall flows and apply ownership using the resolved user/group, preventing repeated permission regressions for custom unprivileged deployments.

Merge requirements satisfied?

[NOTICE] Bug fixes or features added to Salt require tests.

• Docs
• Changelog - https://docs.saltproject.io/en/master/topics/development/changelog.html
• Tests written/updated

Commits signed with GPG?

No