Generate unique ABI names that mostly sort like their package versions

Currently we add a +bpo suffix to the ABI name for bookworm-backports (previously for any -backports suite), which causes the kernel version string to sort higher than the corresponding version in unstable. This is undesirable as it is the opposite of the package version ordering.

Once bookworm enters LTS we will also currently generate the same ABI names from linux in trixie(-security) and linux-6.12 in bookworm-security, although they will not be ABI compatible.

To address both of these issues, set the ABI suffixes:

• +deb12 in bookworm-backports (instead of +bpo)
• +deb13 in trixie
• +deb13 in unstable (to be replaced by +deb14 after trixie release)

Additionally add a serial number suffix whenever we make a second or later upload with the same upstream version and Debian release. This unfortunately isn't guaranteed to sort properly, but such uploads should be rare.

This has been split out from !1481 (merged) and builds on top of that.