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()

src/functionObjects/utilities/vtkWrite/vtkWrite.C

@@ -304,7 +304,7 @@ bool Foam::functionObjects::vtkWrite::write()
         // Prune restart fields
         acceptField.filterKeys
         (
-            [](const word& k){ return k.endsWith("_0"); },
+            [](const word& k){ return k.ends_with("_0"); },
             true // prune
         );