version: rename threadsafe-init to threadsafe

Referring to Daniel's article [1], making the init function thread-safe
was the last bit to make libcurl thread-safe as a whole. So the name of
the feature may as well be the more concise 'threadsafe', also telling
the story that libcurl is now fully thread-safe, not just its init
function. Chances are high that libcurl wants to remain so in the
future, so there is little likelihood of ever needing any other distinct
`threadsafe-<name>` feature flags.

For consistency we also shorten `CURL_VERSION_THREADSAFE_INIT` to
`CURL_VERSION_THREADSAFE`, update its description and reference libcurl's
thread safety documentation.

[1]: https://daniel.haxx.se/blog/2022/06/08/making-libcurl-init-more-thread-safe/

Reviewed-by: Daniel Stenberg
Reviewed-by: Jay Satiro
Closes #8989

lib/version.c

@@ -455,7 +455,7 @@ static curl_version_info_data version_info = {
   | CURL_VERSION_GSASL
 #endif
 #if defined(GLOBAL_INIT_IS_THREADSAFE)
-  | CURL_VERSION_THREADSAFE_INIT
+  | CURL_VERSION_THREADSAFE
 #endif
   ,
   NULL, /* ssl_version */