mkosi: update debian commit reference

* 51cd22f368 Update changelog for 257~rc2-3 release
* 5308c3b905 Backport patch to remove faulty unit test assertion
* b7d805151b Update changelog for 257~rc2-2 release
* 5afc23b288 Backport patch to fix FTBFS due to failing unit test
* 0ca89ce40c Update changelog for 257~rc2-1 release
* f27216d493 Update lintian override to ignore false positive typos
* 2caa74f473 d/rules: adjust blhc override to account for source files being moved
* 6b48328ead systemd-ukify: recommend systemd-repart
* 5e01b67f43 systemd-ukify: downgrade dependency on systemd, not mandatory
* 3a4dd59e41 Install new systemd-keyutil binary in the systemd-repart package
* e64cffab71 Drop all patches, merged upstream
* 0fcef228c7 Update upstream source from tag 'upstream/257_rc2'
* a01322bb29 d/t/control: add more packages to dummy hint-testsuite-triggers

mkosi.images/build/mkosi.conf.d/debian-ubuntu/mkosi.conf

@@ -9,7 +9,7 @@ Environment=
         GIT_URL=https://salsa.debian.org/systemd-team/systemd.git
         GIT_SUBDIR=debian
         GIT_BRANCH=debian/master
-        GIT_COMMIT=48fabbd5d240a70fce6712b6161f29b40b2fc7de
+        GIT_COMMIT=51cd22f3684725a1b199012555e7378f2f468c16
         PKG_SUBDIR=debian
 
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