refactor(web): deprecated APIs to remove in v20

* API endpoints deprecated in v19 (#16351, #16354), to be removed in version 20:
- `keyman.helpURL`
- `keyman.util.rgba()`
- `keyman.util.toFloat()`
- `keyman.util.toNumber()`
- `keyman.util.toNzString()`
* Previously deprecated API endpoints to be removed in version 20:
- `keyman.util.createShim()`
- `keyman.util.showShim()`
- `keyman.util.hideShim()`
- `keyboard.KeyboardID`
* Deprecated IMX functions and endpoints that will be removed in v20:
- `keyman.interface.focusLastActiveElement()`
- `keyman.interface.getLastActiveElement()`
- `keyman.interface.hideHelp()`
- `keyman.interface.showHelp()`
- `keyman.interface.showPinnedHelp()`

* Deprecated IMX functions and endpoints that will be retained for compatibility with legacy IMX keyboards:
- `KeymanWeb.FocusLastActiveElement()`
- `KeymanWeb.GetLastActiveElement()`
- `KeymanWeb.HideHelp()`
- `KeymanWeb.ShowHelp()`
- `KeymanWeb.ShowPinnedHelp()`

* Deprecated functions and endpoints that will be retained for compatibility with legacy keyboards:
- `KeymanWeb.KA()`
- `KeymanWeb.KC()`
- `KeymanWeb.KCM()`
- `KeymanWeb.KDM()`

See-also: #16386

Start by locating the definitions and references for each listed deprecated API and endpoint, then separate the APIs slated for removal from the legacy compatibility APIs that must remain. Done means the v20 removal list is no longer exposed while the listed legacy KeymanWeb APIs remain available for compatibility; verify the affected web tests if present.

Written by the indexing model from the issue text.