Update excon to v0.90.0

This fixes a problem if only `SSL_CERT_DIR` were specified, we would get
an HTTPS failure since the default paths were not used
(https://github.com/excon/excon/pull/772).

This came out of
https://gitlab.com/gitlab-org/omnibus-gitlab/-/issues/6616.

Diff: https://my.diffend.io/gems/excon/0.89.0/0.90.0

Changelog: fixed

Gemfile.lock

@@ -336,7 +336,7 @@ GEM
     ethon (0.15.0)
       ffi (>= 1.15.0)
     eventmachine (1.2.7)
-    excon (0.89.0)
+    excon (0.90.0)
     execjs (2.8.1)
     expression_parser (0.9.0)
     extended-markdown-filter (0.6.0)