`focus-in-hook` obsolete in emacs 27.1

[gonewest818]

Research and deal with this lint error, which appears on the emacs master branch (v. 27.1):

dimmer.el:293:20:Error: `focus-in-hook' is an obsolete variable (as of 27.1); after-focus-change-function

Full test transcript below:

#!/bin/bash -eo pipefail
make lint
emacs --batch -q -l .emacs/init.el -f elisp-lint-files-batch dimmer.el
Loading /root/project/.emacs/.emacs-custom.el (source)...
Loading /root/project/.emacs/dependencies.el (source)...
***************************************************************************
** ELISP-LINT: check dimmer.el...
* Run byte-compile
  INFO     Scraping files for project-autoloads.el... 
Generating autoloads for dimmer.el...
Generating autoloads for dimmer.el...done
  INFO     Scraping files for project-autoloads.el...done
Wrote /root/project/project-autoloads.el
Wrote /root/project/project-autoloads.el
Loading /root/project/project-autoloads.el (source)...

In toplevel form:
dimmer.el:293:20:Error: `focus-in-hook' is an obsolete variable (as of 27.1); after-focus-change-function
* Run check-declare
* Run checkdoc
* Run package-format
* Run indent
Indenting region... 
Indenting region...done
* Run indent-character
* Run fill-column
* Run trailing-whitespace
** ELISP:LINT: dimmer.el FAIL
Makefile:17: recipe for target 'lint' failed
make: *** [lint] Error 1
Exited with code 2