third_party: update libcurl from 8.5.0 to 8.6.0

The patch updates curl module to the version 8.6.0 [1][2] that
brings a number of functional fixes, and updates CMake module for
building curl library.

Changes in CMake module:

- Option `ENABLE_CURL_MANUAL` was added and disabled by default [3]
- Option `BUILD_LIBCURL_DOCS` was added and disabled by default [3]

Previous changelog entry has been removed because duplicate
entries about bumps in release changelog confuses end users.

This bump was blocked by a regression in libcurl [4].

1. https://curl.se/changes.html#8_6_0
2. https://github.com/curl/curl/compare/curl-8_5_0...curl-8_6_0
3. https://github.com/curl/curl/commit/a808aab06851d4364ab1773c664df3d906a497a9
4. https://github.com/curl/curl/commit/b8c003832d730bb2f4b9de4204675ca5d9f7a903

NO_DOC=libcurl submodule bump
NO_TEST=libcurl submodule bump

(cherry picked from commit 00cfc959)