Fix shared runner minutes reset on self-managed

What does this MR do?

When namespace count is lower than 100,000, the :ci_parallel_minutes_reset feature fails with a zero division error due to integer division

This affects most self-managed instances but not GitLab.com which has well over 100,000 namespaces

The fix here is done by using float division to avoid the chance of a ZeroDivisionError in such edge cases

Closes: #330068 (closed)

Screenshots (strongly suggested)

{ … 
  "error_message":"divided by 0",
  "error_class":"ZeroDivisionError",
  "error_backtrace": [
    "ee/app/workers/clear_shared_runners_minutes_worker.rb:21:in `perform'",
    …
  ]
  …
}

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.