Make captcha_modal.vue work in JH

What does this MR do?

Google's reCAPTCHA default endpoint is blocked in china, this MR is a patch to in !64497 (merged), it makes the hardcoded url in frontend file work in JH using gon variable

CC: @qianzhangxa

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Does this MR meet the acceptance criteria?

Conformity

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• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
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• This change is backwards compatible across updates, or this does not apply.

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Security

Does this MR contain changes to processing or storing of credentials or tokens, authorization and authentication methods or other items described in the security review guidelines? If not, then delete this Security section.

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
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