In recaptcha gem version 4.3.1, they are called:
export RECAPTCHA_SITE_KEY= '6Lc6BAAAAAAAAChqRbQZcn_yyyyyyyyyyyyyyyyy'
RECAPTCHA_PUBLIC_KEY is now called RECAPTCHA_SITE_KEY and RECAPTCHA_PRIVATE_KEY is now called RECAPTCHA_SECRET_KEY.
You can set these environment variables in the .profile file in the home folder of your production server.
Since the recaptcha gem does not have any release notes, you have to compare different releases and look at the diff:
Comparing different versions I created a release notes on the wiki: