tools/memory-model: Avoid duplicating herdtools versions

Currently, herdtools version information appears no fewer than three
times in the LKMM source, which is difficult to maintain. This commit
therefore places the required version in one place, namely the
tools/memory-model/README file.

Signed-off-by: Andrea Parri <[e-mail]>
Signed-off-by: Paul E. McKenney <[e-mail]>
Acked-by: Alan Stern <[e-mail]>

tools/memory-model/README

···
 REQUIREMENTS
 ============
 
-Version 7.49 of the "herd7" and "klitmus7" tools must be downloaded
-separately:
+Version 7.52 or higher of the "herd7" and "klitmus7" tools must be
+downloaded separately:
 
   https://github.com/herd/herdtools7
 
 See "herdtools7/INSTALL.md" for installation instructions.
+
+Note that although these tools usually provide backwards compatibility,
+this is not absolutely guaranteed.  Therefore, if a later version does
+not work, please try using the exact version called out above.
 
 
 ==================

tools/memory-model/linux-kernel.cat

···
 // SPDX-License-Identifier: GPL-2.0+
 (*
- * Requires herd version 7.51+6 or higher.
- *
  * Copyright (C) 2015 Jade Alglave <[e-mail]>,
  * Copyright (C) 2016 Luc Maranget <[e-mail]> for Inria
  * Copyright (C) 2017 Alan Stern <[e-mail]>,

tools/memory-model/lock.cat

···
 
 (*
  * Generate coherence orders and handle lock operations
- *
- * Warning: spin_is_locked() crashes herd7 versions strictly before 7.48.
- * spin_is_locked() is functional from herd7 version 7.49.
  *)
 
 include "cross.cat"