Use parent integration settings when reverting from custom settings

What does this MR do?

When switching from custom settings to inherited settings, we rely on the frontend to fill in the inherited values. For encrypted fields we can't do that, so if one of them is required it's not possible to revert an integration from custom settings back to default settings.

This is fixed by ignoring the settings sent by the frontend and instead propagating the values from the parent integration.

Fixes #336634 (closed), and other affected integrations (Bamboo, Jenkins, Jira, Teamcity).

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are 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.
• This change is backwards compatible across updates, or this does not apply.

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.

Related to #336634 (closed)