ENH: rationalize some string methods.

- silently deprecate 'startsWith', 'endsWith' methods
  (added in 2016: 2b14360662), in favour of
  'starts_with', 'ends_with' methods, corresponding to C++20 and
  allowing us to cull then in a few years.

- handle single character versions of starts_with, ends_with.

- add single character version of removeEnd and silently deprecate
  removeTrailing which did the same thing.

- drop the const versions of removeRepeated, removeTrailing.
  Unused and with potential confusion.

STYLE: use shrink_to_fit(), erase()

applications/utilities/postProcessing/graphics/PVReaders/foamReader/library/vtkPVFoam.C

@@ -678,7 +678,7 @@ void Foam::vtkPVFoam::Update
         // Suppress caching of Lagrangian since it normally always changes.
         cachedVtp_.filterKeys
         (
-            [](const word& k){ return k.startsWith("lagrangian/"); },
+            [](const word& k){ return k.starts_with("lagrangian/"); },
             true // prune
         );
     }

applications/utilities/postProcessing/graphics/PVReaders/foamReader/library/vtkPVFoamMesh.C

@@ -179,7 +179,7 @@ void Foam::vtkPVFoam::convertMeshPatches()
             }
         }
 
-        if (longName.startsWith("group/"))
+        if (longName.starts_with("group/"))
         {
             // Patch group. Collect patch faces.