Switch the default config option values for recaptcha_sitekey_class and recaptcha_form_field (#2939)

Attempting to use the [web auth fallback
mechanism](https://spec.matrix.org/v1.5/client-server-api/#fallback) for
Google ReCAPTCHA with the default setting for
`client_api.recaptcha_sitekey_class` of "g-recaptcha-response" results
in no captcha being rendered:


![image](https://user-images.githubusercontent.com/1342360/212482321-14980045-6e20-4d59-adaa-59a01ad88367.png)

I cross-checked the captcha code between [dendrite.matrix.org's fallback
page](https://dendrite.matrix.org/_matrix/client/r0/auth/m.login.recaptcha/fallback/web?session=asdhjaksd)
and [matrix-client.matrix.org's
one](https://matrix-client.matrix.org/_matrix/client/r0/auth/m.login.recaptcha/fallback/web?session=asdhjaksd)
(which both use the same captcha public key) and noticed a discrepancy
in the `class` attribute of the div that renders the captcha.
[ReCAPTCHA's docs
state](https://developers.google.com/recaptcha/docs/v3#automatically_bind_the_challenge_to_a_button)
to use "g-recaptcha" as the class for the submit button.

I noticed this when user `@parappanon:parappa.party` reported that they
were also seeing no captcha being rendered on their Dendrite instance.
Changing `client_api.recaptcha_sitekey_class` to "g-recaptcha" caused
their captcha to render properly as well.

There may have been a change in the class name from ReCAPTCHA v2 to v3?
The [docs for
v2](https://developers.google.com/recaptcha/docs/display#auto_render)
also request one uses "g-recaptcha" though.

Thus I propose changing the default setting to unbreak people's
recaptcha auth fallback pages. Should fix dendrite.matrix.org as well.

setup/config/config_clientapi.go

@@ -85,10 +85,10 @@ func (c *ClientAPI) Verify(configErrs *ConfigErrors, isMonolith bool) {
 			c.RecaptchaApiJsUrl = "https://www.google.com/recaptcha/api.js"
 		}
 		if c.RecaptchaFormField == "" {
-			c.RecaptchaFormField = "g-recaptcha"
+			c.RecaptchaFormField = "g-recaptcha-response"
 		}
 		if c.RecaptchaSitekeyClass == "" {
-			c.RecaptchaSitekeyClass = "g-recaptcha-response"
+			c.RecaptchaSitekeyClass = "g-recaptcha"
 		}
 		checkNotEmpty(configErrs, "client_api.recaptcha_public_key", c.RecaptchaPublicKey)
 		checkNotEmpty(configErrs, "client_api.recaptcha_private_key", c.RecaptchaPrivateKey)