Clean up CHANGELOG_PENDING after v3.21.0 release
### Improvements
-- [engine] - Interpret `pluginDownloadURL` as the provider host url when
- downloading plugins.
- [#8544](https://github.com/pulumi/pulumi/pull/8544)
-
-- [sdk/dotnet] - `InputMap` and `InputList` can now be initialized
- with any value that implicitly converts to the collection type.
- These values are then automatically appended, for example:
-
- var list = new InputList<string>
- {
- "V1",
- Output.Create("V2"),
- new[] { "V3", "V4" },
- new List<string> { "V5", "V6" },
- Output.Create(ImmutableArray.Create("V7", "V8"))
- };
-
- This feature simplifies the syntax for constructing resources and
- specifying resource options such as the `DependsOn` option.
-
- [#8498](https://github.com/pulumi/pulumi/pull/8498)
-
### Bug Fixes
-
-- [sdk/python] - Fixes an issue with stack outputs persisting after
- they are removed from the Pulumi program
- [#8583](https://github.com/pulumi/pulumi/pull/8583)
-
-- [auto/*] - Fixes `stack.setConfig()` breaking when trying to set
- values that look like flags (such as `-value`)
- [#8518](https://github.com/pulumi/pulumi/pull/8614)
-
-- [sdk/dotnet] - Don't throw converting value types that don't match schema
- [#8628](https://github.com/pulumi/pulumi/pull/8628)
-
-- [sdk/{go,nodejs,dotnet,python}] - Compute full set of aliases when both parent and child are aliased.
- [#8627](https://github.com/pulumi/pulumi/pull/8627)
-
-- [cli/import] - Fix import of resource with non-identifier map keys
- [#8645](https://github.com/pulumi/pulumi/pull/8645)
-
-- [backend/filestate] - Allow preview on locked stack
- [#8642](https://github.com/pulumi/pulumi/pull/8642)
