Use standard timestamp format for application logs

What does this MR do?

Original issue: #43422 (closed)

This MR, changes the timestamp format in application logs, to use UTC and ISO8601.3, which is the standard format we want to use in all our logging.

This is part of the on-going work on standardizing timestamps across logs.

This MR builds on the work done to introduce structured logging in application logs here and here.

Screenshots

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

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