mint-release-upgrade: point upgrade keeps on removing synaptic

Describe the bug
in the past, mint's fork of synaptic was part of the base system and used by other programs. eventually, dependency on synaptic was eliminated and mint's fork was retired. the point upgrade that retired mint's synaptic uninstalled the package.

some users chose to have ubuntu's synaptic installed instead for reasons, and the point upgrade blindly uninstalled that too; but that was not really important as it affected only non-standard setups.

more importantly, mint-release-upgrade keeps on uninstalling synaptic on each upgrade for no reason. once mint's fork was out of the picture with the first upgrade, there is no point in keeping on removing the package (which now must be ubuntu's, or else some orphan). if the package is there it is because the user manually installed and point upgrades should not touch it.

To Reproduce
Steps to reproduce the behavior:

1. setup mint 22.2
2. install ubuntu's synaptic
3. upgrade to mint 22.3
4. synaptic is gone

Expected behavior
synaptic should not be touched by recent point upgrades.

Distribution:

• Linux Mint
• LMDE
• Other (please specify)

Software version:
7.1.1

Start by reproducing the issue with mint 22.2, Ubuntu's synaptic package, and the upgrade to mint 22.3. Trace the mint-release-upgrade package-removal handling and verify that a manually installed synaptic package remains untouched during recent point upgrades.