Create encrypted application setting for product analytics clickhouse

What does this MR do and why?

• Adds new encrypted attribute product_analytics_clickhouse_connection_string to application settings. Also copies the value from clickhouse_connection_string to the newly created column. In a follow-up MR, we will ignore the column in a future release.

How to set up and validate locally

• Ensure that Gitlab::CurrentSettings.clickhouse_connection_string is defined. (Doesn't matter what it is.)
• Run the migrations.
• Assert that Gitlab::CurrentSettings.product_analytics_clickhouse_connection_string is the same as Gitlab::CurrentSettings.clickhouse_connection_string.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Relates to #384185 (closed)