Hi @pzanto,
Since CF7 v5.1 reCAPTCHA v2 was replace for v3. However there is an easy way to restore reCaptcha v2 in the latest version:
1. Update CF7 to the lastest version
2. Download CF7 v5.0.5.
3. Replace the recaptcha.php file of your WordPress installation located in “[plugin directory]/contact-form-7/modules/” with the file with the same name of the v5.0.5 package you downloaded in the step 1.
4. In the dashboad, navigate into to Contact > Integration, and choose “Configure Keys” in the “reCAPTCHA” section. If you already have configured the keys, verify those keys are for the v2 (if you don’t see reCAPTCHA in the frontend, you have the v3). To fix it, register a new site with reCAPTCHA v2 and then back to Contact > Integration and choose “Reset keys”, write the new keys and save. You should see reCAPTCHA working on frontend.
I hope this can help you, cheers!