Disable usr-merge support for older Debian/Ubuntu releases that didn't have the merged /usr directories scheme.

Author: ximon18

Cargo.toml

@@ -67,7 +67,19 @@ maintainer-scripts = "pkg/debian"
 changelog = "target/debian/changelog" # this will be generated by the pkg workflow
 copyright = "Copyright (c) 2025, NLnet Labs. All rights reserved."
 conf-files = ["/etc/kmip2pkcs11/config.toml"]
-systemd-units = { unit-name = "kmip2pkcs11", unit-scripts = "pkg/common", enable = false, usr-merge = true } # usr-merge is needed for Trixie and later
+systemd-units = { unit-name = "kmip2pkcs11", unit-scripts = "pkg/common", enable = false, usr-merge = true } # usr-merge is needed for Bookworm/Noble and later
+
+[package.metadata.deb.variants.debian-buster]
+systemd-units = { unit-name = "kmip2pkcs11", unit-scripts = "pkg/common", enable = false, usr-merge = false }
+
+[package.metadata.deb.variants.debian-bullseye]
+systemd-units = { unit-name = "kmip2pkcs11", unit-scripts = "pkg/common", enable = false, usr-merge = false }
+
+[package.metadata.deb.variants.ubuntu-focal]
+systemd-units = { unit-name = "kmip2pkcs11", unit-scripts = "pkg/common", enable = false, usr-merge = false }
+
+[package.metadata.deb.variants.ubuntu-jammy]
+systemd-units = { unit-name = "kmip2pkcs11", unit-scripts = "pkg/common", enable = false, usr-merge = false }
 
 [package.metadata.generate-rpm]
 assets = [