Salesforce login button text change

What does this MR do and why?

Solves https://gitlab.com/gitlab-org/gitlab/-/issues/422028

Salesforce login button text change

Salesforce cloud logo cannot be used and hence the button text is changed to indicate user can sign in via Salesforce.

Changelog: changed

Screenshots or screen recordings

How to set up and validate locally

1. Enable Salesforce provider in gitlab.yml file under omniauth-providers
2. Observe the change in Sign in button on login screen

MR acceptance checklist

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