Update libcurl

Engine/lib/curl/docs/ALTSVC.md

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+# Alt-Svc
+
+curl features support for the Alt-Svc: HTTP header.
+
+## Enable Alt-Svc in build
+
+`./configure --enable-alt-svc`
+
+(enabled by default since 7.73.0)
+
+## Standard
+
+[RFC 7838](https://datatracker.ietf.org/doc/html/rfc7838)
+
+# Alt-Svc cache file format
+
+This a text based file with one line per entry and each line consists of nine
+space separated fields.
+
+## Example
+
+    h2 quic.tech 8443 h3-22 quic.tech 8443 "20190808 06:18:37" 0 0
+
+## Fields
+
+1. The ALPN id for the source origin
+2. The host name for the source origin
+3. The port number for the source origin
+4. The ALPN id for the destination host
+5. The host name for the destination host
+6. The host number for the destination host
+7. The expiration date and time of this entry within double quotes. The date format is "YYYYMMDD HH:MM:SS" and the time zone is GMT.
+8. Boolean (1 or 0) if "persist" was set for this entry
+9. Integer priority value (not currently used)
+
+# TODO
+
+- handle multiple response headers, when one of them says `clear` (should
+  override them all)
+- using `Age:` value for caching age as per spec
+- `CURLALTSVC_IMMEDIATELY` support