shorten TAMRequiredError error msg

Users using recently created repos (after borg 1.0.9) or
who followed the upgrade procedure of 1.0.9 or >1.2.4 will
never see this error msg.

So, have it as short as and similar to the ArchiveTAMRequiredError.

docs/internals/frontends.rst

@@ -689,11 +689,6 @@ Errors
         Data integrity error: {}
     TAMRequiredError rc: 98 traceback: yes
         Manifest is unauthenticated, but it is required for this repository.
-
-        This either means that you are under attack, or that you modified this repository
-        with a Borg version older than 1.0.9 after TAM authentication was enabled.
-
-        In the latter case, use "borg upgrade --tam --force '{}'" to re-authenticate the manifest.
     TAMUnsupportedSuiteError rc: 99 traceback: yes
         Could not verify manifest: Unsupported suite {!r}; a newer version is needed.
 

src/borg/crypto/key.py

@@ -90,11 +90,6 @@ class RepoKeyNotFoundError(Error):
 class TAMRequiredError(IntegrityError):
     __doc__ = textwrap.dedent("""
     Manifest is unauthenticated, but it is required for this repository.
-
-    This either means that you are under attack, or that you modified this repository
-    with a Borg version older than 1.0.9 after TAM authentication was enabled.
-
-    In the latter case, use "borg upgrade --tam --force '{}'" to re-authenticate the manifest.
     """).strip()
     traceback = True
     exit_mcode = 98