Add soft limit daily phone verification attribute to application setting

What does this MR do and why?

The work in this MR solves for task: Convert soft_phone_verification_transactions_limit to an application setting

We are adding a JSONB attribute - soft_phone_verification_transactions_limit in the ApplicationSetting model under identity_verification_settings column with default value set as 16_000

MR acceptance checklist

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How to set up and validate locally

1. In rails console set the value of soft_phone_verification_transactions_limit to 2 by running following command

ApplicationSetting.last.update(soft_phone_verification_transactions_limit: 2)

2. Now start making calls to throttled? method of ::Gitlab::ApplicationRateLimiter multiple times. Expectation is for the first two times it will return false as the count is still under the limit but in the third time it will return true.

::Gitlab::ApplicationRateLimiter.throttled?(:soft_phone_verification_transactions_limit, scope: nil)
=> false

::Gitlab::ApplicationRateLimiter.throttled?(:soft_phone_verification_transactions_limit, scope: nil)
=> false

# 3rd request
::Gitlab::ApplicationRateLimiter.throttled?(:soft_phone_verification_transactions_limit, scope: nil)
=> true