libcurl-gnutls: update to 8.20.0

libcurl-gnutls: update to 8.20.0

Tracks upstream curl 8.20.0 (April 2026 release).

Changes since 8.14.1 cover six upstream releases (8.15.0 - 8.20.0).
Highlights:
 * async-thrdd: use thread queue for resolving
 * build: make NTLM disabled by default
 * lib: add thread pool and queue
 * lib: drop support for < c-ares 1.16.0
 * lib: make SMB support opt-in
 * multi.h: add CURLMNWC_CLEAR_ALL
 * rtmp: drop support
 * cmake: drop support for CMake 3.17 and older
 * Various TLS, HTTP/3, altsvc and resolver bug fixes.

Link: https://curl.se/changes.html#8_20_0
Link: https://github.com/curl/curl/blob/curl-8_20_0/RELEASE-NOTES
Signed-off-by: Daniel Golle <redacted>