Upgrade to libcURL v8.15.0

[jviotti]

See https://github.com/curl/curl/releases/tag/curl-8_15_0. Using https://github.com/sourcemeta/vendorpull by modifying DEPENDENCIES and running ./vendor/vendorpull/pull curl.

[jviotti]

Main challenge is that they removed BearSSL support :(

[jviotti]

Seems like we could use https://github.com/Mbed-TLS/mbedtls

[jviotti]

Also see https://daniel.haxx.se/blog/2025/06/11/dropping-some-tls-laggards/