Add block_changes_at to License type in GraphQL

What does this MR do?

Related to #330958

This MR adds the block_changes_at attribute to the LicenseType GraphQL type, making it available to GraphQL queries on currentLicense or licenseHistoryEntries. See the examples queries below.

Local testing

1. Login to your local GDK with root credentials.
2. Navigate to http://localhost:3000/-/graphql-explorer
3. Query the current license with the following:

query {
  currentLicense {
    id
    type
    plan
    name
    email
    company
    startsAt
    expiresAt
    lastSync
    blockChangesAt
  }
}

1. and query the license history with:

query {
  licenseHistoryEntries {
    nodes {
      id
      type
      plan
      name
      email
      company
      startsAt
      expiresAt
      blockChangesAt
    }
  }
}

Does this MR meet the acceptance criteria?

Conformity

• I have included a changelog entry, or it's not 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.

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