Update BoringSSL to 0226f30467f540a3f62ef48d453f93927da199b6

[gwynne]

Fix multiple issues in the BoringSSL vendoring script and update BoringSSL to 0226f30467f540a3f62ef48d453f93927da199b6

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

Motivation:

The vendor-boringssl.sh script is currently broken in a number of
ways, both in general and relative to the latest BoringSSL version:

• --enable-test-discovery is still in use
• The Linux Swift SDKs being generated are out of date (5.10-jammy)
• The script does not prefix exported C++ symbols
• Invoking the script without a BORINGSSL_REVISION results in an
  unconditional error
• The PATTERNS list is out of date for the latest BoringSSL
• The latest BoringSSL contains a bug which fails to apply extern "C"
  to two exported symbols, allowing for collisions.

In addition, the latest BoringSSL has made BN_MONT_CTX opaque,
necessitating that it be referenced with OpaquePointer rather than
UnsafePointer<>.

Modifications:

The following changes are included:

• --enable-test-discovery has been removed
• vendor-boringssl.sh and generate-linux-sdks.sh have been updated
  to use Swift 6.1.2 SDKs built for Ubuntu Noble
• The mangle_cpp_structures function from swift-nio-ssl's version
  of vendor-boringssl.sh has been copied over (thanks @Lukasa!)
• BORINGSSL_REVISION is now allowed to be an empty string
• The PATTERNS array has been updated
• A patch has been added which applies extern "C" to the functions
  which should have it
• s/UnsafePointer<BN_MONT_CTX>/OpaquePointer/g
• Updated CMakeLists.txt per update-cmake-lists.sh
• Update BoringSSL to
  0226f30467f540a3f62ef48d453f93927da199b6

Result:

Vendoring the latest BoringSSL will work again.
The latest BoringSSL is vendored.