Fix notes_data so the initial value is in microseconds

What does this MR do?

Without this change, we always start polling for updated notes far back in time, as the value is interpreted as microseconds, instead of seconds.

This change was introduced as part of !34628 (diffs) - regardless of the feature flag, last_fetched_at is a value that is set and read in microseconds.

Once the paginated notes endpoint is used to populate discussions from the beginning of time, rather than time=now, we'd want to set this value to 0 (or do away with it altogether?), but until then, fixing this will drastically reduce the amount of data we pull from the notes endpoint.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• 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

Part of #209784 (closed)