Rename sprints table to iterations

What does this MR do?

Renames sprints table to iterations so that it would match the terminology used as well as code usages, including model name itself.

Up

== 20201117123226 RenameSprintsTableToIterations: migrating ===================
-- rename_table(:sprints, :iterations)
   -> 0.0287s
== 20201117123226 RenameSprintsTableToIterations: migrated (0.0288s) ==========

Down

== 20201117123226 RenameSprintsTableToIterations: reverting ===================
-- rename_table(:iterations, :sprints)
   -> 0.0173s
== 20201117123226 RenameSprintsTableToIterations: reverted (0.1093s) ==========

Runtime on database-lab

exec ALTER TABLE "sprints" RENAME TO "iterations"
Session: joe-bupvvh8350j13v2kl740
The query has been executed. Duration: 15.000 ms (edited) 

renames sprints table to iterations, so that it is more consistent with the terminology we use as well as model name and multiple code references.

re #232899

Screenshots (strongly suggested)

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