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/lagrangian/vtkCloud/vtkCloud.C

@@ -256,7 +256,7 @@ bool Foam::functionObjects::vtkCloud::writeCloud
     (
         [](const word& k)
         {
-            return k.startsWith("position") || k.startsWith("coordinate");
+            return k.starts_with("position") || k.starts_with("coordinate");
         },
         true  // prune
     );

src/functionObjects/utilities/ensightWrite/ensightWrite.C

@@ -226,7 +226,7 @@ bool Foam::functionObjects::ensightWrite::write()
     // Prune restart fields
     acceptField.filterKeys
     (
-        [](const word& k){ return k.endsWith("_0"); },
+        [](const word& k){ return k.ends_with("_0"); },
         true // prune
     );
 

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
         );