Use SecKeyGetBlockSize instead of kSecAttrKeySizeInBits

[dnadoba]

Checklist

• I've run tests to see all new and existing tests pass
• I've followed the code style of the rest of the project
• I've read the Contribution Guidelines
• I've updated the documentation if necessary

If you've made changes to gyb files

• I've run .script/generate_boilerplate_files_with_gyb and included updated generated files in a commit of this pull request

Motivation:

Security.framework has change to return the exact bit count of the key size instead of the block size in bytes in macOS 14 beta and aligned releases. However, BoringSSL and the previous implementation relyed on that.
SecKeyCopyAttributes is also rather expensive to call and keySizeInBits is accessed immediately (twice) after initialisation.

Modifications:

call SecKeyGetBlockSize(_:)`` instead of looking up the kSecAttrKeySizeInBits` in the attributes dictionary.

Result:

test pass again and allocations are reduced.

[Lukasa]

@swift-server-bot add to allowlist