Enable packages_nuget_new_package_file_updater by default

🛶 Context

The feature flag packages_nuget_new_package_file_updater controls how NuGet package uploads are processed by a background job. This refactoring was need to fix an issue when uploading large files.

See #321361 (comment 622762440) and !66728 (merged) for all the details.

This feature has been rolled out on gitlab.com without any hiccup. See the rollout issue: #336511 (closed).

No errors detected on this dashboard (internal):

It's time to enable the feature flag by default.

We're not removing the feature flag (and related code) yet because self-managed users might have a rather different configuration for object storage than gitlab.com. They can thus experience issues even though we tested several different configurations. Keeping the feature flag is a safety net for them: if they hit an issue with this part, they can simply disable it.

🔬 What does this MR do?

• Enable packages_nuget_new_package_file_updater by default.

🖼 Screenshots or Screencasts (strongly suggested)

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🛀 How to setup and validate locally (strongly suggested)

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⛓ 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.

Security

Does this MR contain changes to processing or storing of credentials or tokens, authorization and authentication methods or other items described in the security review guidelines? If not, then delete this Security section.

• [-] 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