Replace SHA-1 fingerprints with SHA-256 fingerprints

[dtivel]

SHA-1 fingerprints should not be used. Use SHA-256 fingerprints instead. This is for SDL compliance.

From https://github.com/search?q=repo%3ANuGet%2FNuGetGallery%20SHA-1&type=code

• https://github.com/NuGet/NuGetGallery/blob/27cbca36096af5897d3d034ec495c37cd110a1c6/src/NuGetGallery/Views/Packages/_ValidationIssue.cshtml#L56
• https://github.com/NuGet/NuGetGallery/blob/27cbca36096af5897d3d034ec495c37cd110a1c6/src/NuGetGallery.Core/Extensions/ValidationIssueExtensions.cs#L55
• https://github.com/NuGet/NuGetGallery/blob/27cbca36096af5897d3d034ec495c37cd110a1c6/tests/NuGetGallery.Facts/Infrastructure/Mail/MarkdownMessageServiceFacts.cs#L1375
• https://github.com/NuGet/NuGetGallery/blob/27cbca36096af5897d3d034ec495c37cd110a1c6/tests/NuGetGallery.Facts/Infrastructure/Mail/MarkdownMessageServiceFacts.cs#L2318

[joelverhagen]

Related: https://github.com/NuGet/NuGetGallery/issues/9523.

[mariaghiondea]

Related internal item: Task 2046665: Use approved crypto libraries for the supported platform