Remove not_null constraint on project_fingerprint for `security_findings`

What does this MR do?

We are in the process of deprecating the usage of the project_fingerprint attribute of the findings. As there is a null constraint associated to project_fingerprint, we are facing difficulties to remove the use of project_fingerprint from code without removing this constraint. We already introduced uuid which will be used instead of project_fingerprint. This is one step further to deprecate project_fingerprint.

Database Review

Up

== 20210713223941 RemoveNullConstraintFromSecurityFindings: migrating =========
-- change_column_null(:security_findings, :project_fingerprint, true)
   -> 0.0050s
== 20210713223941 RemoveNullConstraintFromSecurityFindings: migrated (0.0050s) 

Down

== 20210713223941 RemoveNullConstraintFromSecurityFindings: reverting =========
== 20210713223941 RemoveNullConstraintFromSecurityFindings: reverted (0.0000s) 

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• 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)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Related to #335700 (closed)