Fix: Accessibility Issues #2624 (#2642)

### Changes

This PR has required code change to address the issue raised
[here](#2624).
Fixes #2624

### References
#2624

### Testing

I have tested the implementation using a [sample React
app](https://github.com/auth0-samples/auth0-react-samples), following
the steps provided to reproduce the issue. Please find the attached
screen recording demonstrating the implementation details.

* [x] This change adds unit test coverage
* [ ] This change adds integration test coverage
* [x] This change has been tested on the latest version of the
platform/language

### Checklist

* [x] I have read the [Auth0 general contribution
guidelines](https://github.com/auth0/open-source-template/blob/master/GENERAL-CONTRIBUTING.md)
* [x] I have read the [Auth0 Code of
Conduct](https://github.com/auth0/open-source-template/blob/master/CODE-OF-CONDUCT.md)
* [x] All code quality tools/guidelines have been run/followed
* [x] All relevant assets have been compiled

Author: ankita10119

src/__tests__/field/__snapshots__/social_buttons_pane.test.jsx.snap

@@ -32,6 +32,7 @@ exports[`SocialButtonsPane calls signUpError when isSignUp===true and terms were
 exports[`SocialButtonsPane renders correctly 1`] = `
 <div
   className="auth-lock-social-buttons-pane"
+  tabIndex={0}
 >
   <div
     className="auth0-lock-social-buttons-container"
@@ -61,6 +62,7 @@ exports[`SocialButtonsPane renders correctly 1`] = `
 exports[`SocialButtonsPane shows header when instructions are available 1`] = `
 <div
   className="auth-lock-social-buttons-pane"
+  tabIndex={0}
 >
   <p>
     instructions
@@ -93,6 +95,7 @@ exports[`SocialButtonsPane shows header when instructions are available 1`] = `
 exports[`SocialButtonsPane shows loading when showLoading === true 1`] = `
 <div
   className="auth-lock-social-buttons-pane"
+  tabIndex={0}
 >
   <div
     className="auth0-lock-social-buttons-container"

src/field/social/social_buttons_pane.jsx

@@ -57,7 +57,7 @@ export default class SocialButtonsPane extends React.Component {
     );
 
     return (
-      <div className="auth-lock-social-buttons-pane">
+      <div className="auth-lock-social-buttons-pane" tabIndex={0}>
         {header}
         <div className="auth0-lock-social-buttons-container">{buttons}</div>
         {loading}