[core] Deprecate TList(TObject*) for removal

This ctor has been "deprecated for 24 years" (#15451 (comment))
A discussion in #15451 concluded to use the modern deprecation procedure
to properly announce its final removal.

Author: dennisklein

README/ReleaseNotes/v632/index.md

@@ -25,6 +25,7 @@ The following people have contributed to this new version:
  Enrico Guiraud, CERN/EP-SFT and Princeton,\
  Jonas Hahnfeld, CERN/EP-SFT,\
  Akeem Hart, Queen Mary University of London/DUNE and MINERvA,\
+ Dennis Klein, GSI,\
  Sergey Linev, GSI,\
  Pere Mato, CERN/EP-SFT,\
  Lorenzo Moneta, CERN/EP-SFT,\
@@ -46,6 +47,7 @@ The following people have contributed to this new version:
 - ROOT does not longer support Python 2. The minimum required Python version to build ROOT is 3.8.
 - Support for wildcard imports like `from ROOT import *` is dropped from PyROOT
 - Support for external (ie. non-builtin) libAfterImage is now deprecated and it will be removed in next release 6.34.
+- The `TList::TList(TObject*)` constructor is deprecated and will be removed in ROOT 6.34
 
 ## Core Libraries
 

core/cont/inc/TList.h

@@ -70,7 +70,9 @@ friend  class TListIter;
    typedef TListIter Iterator_t;
 
    TList() : fAscending(kTRUE) { }
-   TList(TObject *) : fAscending(kTRUE) { } // for backward compatibility, don't use
+
+   TList(TObject *) R__DEPRECATED(6, 34, "The argument is ignored. Use the default constructor TList::TList().") : fAscending(kTRUE) { } // for backward compatibility, don't use
+
    virtual           ~TList();
    void              Clear(Option_t *option="") override;
    void              Delete(Option_t *option="") override;