Fix `StoreReportService` by falling back to find by location approach

What does this MR do?

Related to https://sentry.gitlab.net/gitlab/gitlabcom/issues/2739025

For more information: #276498 (comment 643531295)

Why this change?

Previously we ran a background migration to re-populate the UUID values for vulnerability findings which calculated the wrong UUID values for all existing records. Therefore, whenever we try to find a vulnerability finding by UUID, we can't find it because the wrong UUID values are stored in the database. As we can't find the vulnerability findings, we try to create them and a compound unique constraint fails on the DB layer. This change is just a workaround to use the old-fashion find_params approach to find the existing vulnerability findings if we can't find any by using the UUIDs before we try to create a new one.

find_finding_by_uuid OR find_finding_by_signatures OR finding_finding_by_find_params OR create_new_finding

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.

Availability and Testing

• [-] I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• [-] I have tested this MR in all supported browsers, or it's not needed.
• [-] I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Security

Does this MR contain changes to processing or storing of credentials or tokens, authorization and authentication methods or other items described in the security review guidelines? If not, then delete this Security section.

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team