MR Analytics - Fix throughput chart date range calculation

What does this MR do?

Fixes a bug which caused the MR Analytics Throughput Chart date range calculations to not be accurate.

How to test:

1. Make sure you have starter plan.
2. Enable the feature: Feature.enable(:project_merge_request_analytics)
3. Navigate to the page: some project / analytics / Merge Requests

Before

Sep_2019: mergeRequests(first: 0, mergedBefore: "2019-10-01", mergedAfter: "2019-10-01”)
Oct_2019: mergeRequests(first: 0, mergedBefore: "2019-12-01", mergedAfter: "2019-10-01") 
Nov_2019: mergeRequests(first: 0, mergedBefore: "2019-12-01", mergedAfter: "2019-12-01")
Dec_2019: mergeRequests(first: 0, mergedBefore: "2020-01-01", mergedAfter: "2019-12-01")
Jan_2020: mergeRequests(first: 0, mergedBefore: "2020-03-01", mergedAfter: "2020-01-01") 
Feb_2020: mergeRequests(first: 0, mergedBefore: "2020-03-01", mergedAfter: "2020-03-01") 
Mar_2020: mergeRequests(first: 0, mergedBefore: "2020-05-01", mergedAfter: "2020-03-01")
Apr_2020: mergeRequests(first: 0, mergedBefore: "2020-05-01", mergedAfter: "2020-05-01")
May_2020: mergeRequests(first: 0, mergedBefore: "2020-07-01", mergedAfter: "2020-05-01")
Jun_2020: mergeRequests(first: 0, mergedBefore: "2020-07-01", mergedAfter: "2020-07-01")
Jul_2020: mergeRequests(first: 0, mergedBefore: "2020-08-01", mergedAfter: "2020-07-01") 
Jul_2020: mergeRequests(first: 0, mergedBefore: "2020-08-01", mergedAfter: "2020-07-01")
Aug_2020: mergeRequests(first: 0, mergedBefore: "2020-10-01", mergedAfter: "2020-08-01")

After

Sep_2019: mergeRequests(first: 0, mergedBefore: "2019-10-01", mergedAfter: "2019-09-01”) 
Oct_2019: mergeRequests(first: 0, mergedBefore: "2019-11-01", mergedAfter: "2019-10-01")
Nov_2019: mergeRequests(first: 0, mergedBefore: "2019-12-01", mergedAfter: "2019-11-01")
Dec_2019: mergeRequests(first: 0, mergedBefore: "2020-01-01", mergedAfter: "2019-12-01”) 
Jan_2020: mergeRequests(first: 0, mergedBefore: "2020-02-01", mergedAfter: "2020-01-01") 
Feb_2020: mergeRequests(first: 0, mergedBefore: "2020-03-01", mergedAfter: "2020-02-01”)
Mar_2020: mergeRequests(first: 0, mergedBefore: "2020-04-01", mergedAfter: "2020-03-01")
Apr_2020: mergeRequests(first: 0, mergedBefore: "2020-05-01", mergedAfter: "2020-04-01")
May_2020: mergeRequests(first: 0, mergedBefore: "2020-06-01", mergedAfter: "2020-05-01")
Jun_2020: mergeRequests(first: 0, mergedBefore: "2020-07-01", mergedAfter: "2020-06-01")
Jul_2020: mergeRequests(first: 0, mergedBefore: "2020-08-01", mergedAfter: "2020-07-01")
Aug_2020: mergeRequests(first: 0, mergedBefore: "2020-09-01", mergedAfter: "2020-08-01")

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
• [-] 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

Related issue

#243446 (closed)