Update to ng-dynamic-forms v15 and rxjs v7

[tdonohue]

Description

Discovered in #1567. There's a newer version of ng-dynamic-forms available to support Angular 13. However, it requires also upgrading to rxjs v7.

@ybnd analyzed the effort here and noted:

• Upgrading only rxjs to v7 results in ~200 test failures, a good number of which fail due pretty uninformatively within rxjs code, no DSpace in the trace at all -- this will take me a while to figure out
• ng-dynamic-forms@15 depends on rxjs@7, so it would be best to do both at the same time
• When upgrading both, as well as other mismatched peer dependencies that come up ~ rxjs@7 reduces the amount of failing tests to 11, so it would definitely be more manageable to upgrade all of these at once.

Estimated at 8hrs by @artlowel

Related work
Follow-up work for #1567