Restore DBX binary paths that were removed

* The recent reorganisation of the signed DBX binaries in 92b433e had
  the unfortunate effect of breaking downstream workflows, that relied on the presence of the
  DXBUpdate.bin binaries in their previous location/names.
* To give some time for these workflow to be updated, the KEK 2011 signed objects that existed
  at the time of the removal are restored, while being being flagged as obsolete and planned
  for deletion at a later date as it will not be updated further. In the depreciation notice
  we also point to using the new .auth2 binaries from either SignedByKEK2011 or SignedByKEK2023.
* Note that it was not possible to use symlinks, as the GitHub API does not resolve symlinks
  URLs (the download URL it provides is the one of the symlink text), and also, because some
  workflows depend on being able to read the date from the commit that modifies the target of
  the symlink, whereas the GitHub API only ever returns the date the symlink was created.
* However, we also additionally create a SignedByKEKLatest symlink, that currently points to
  SignedByKEK2023, so that workflows that want to anticipate KEK 2023 expiration in 11 years
  years can start using that symlinked reference as a semi-immutable means of accessing the
  latest DBX binaries.

Author: pbatard

PostSignedObjects/DBX/README.md

@@ -0,0 +1,9 @@
+# Contents of the DBX Folder
+
+This folder contains the last auth2 DBX binaries that were signed using KEK 2011.
+
+Because KEK 2011 has now expired, and newer DBX binaries are signed with KEK 2023, this
+folder and its content should be considered obsolete and **will be deleted in 2027**.
+
+If you were referencing content from this folder directly, you should move to using the new
+`.auth2` binaries from either the `SignedByKEK2011` or `SignedByKEK2023` folders.

PostSignedObjects/DBX/amd64/DBXUpdate.bin

@@ -0,0 +1 @@
+SignedByKEK2023