change(scan): Standardise on the shielded-scan feature name

[teor2345]

Motivation

We're using both the shielded-scan and zebra-scan feature names. Due to some cargo automatic feature changes in 2024, we should avoid using feature names that are the same as crates. (They might work differently soon.)

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Specifications

rust-lang/cargo#12173

Solution

Just use the shielded-scan feature

Testing

The existing tests cover this, and using this feature name might activate more tests.

Review

This is a routine cleanup.

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[teor2345]

This is also a test for ticket #8037, which is an admin repository variable change to enable the shielded-scan tests in CI.

[oxarbitrage]

Yea, i noticed this when reviewing #8036 but didn't wanted to add more stuff there. Thanks for the upgrade!