upgrade libcurl for security vulnerabilities.

[lalitb]

Upgrade bazel build to use the latest libcurl version v8.4.0 (to be released on Oct 11, 2023) for below two security vulnerabilities:

CVE-2023-38545, a high severity flaw that affects both the libcurl library and the curl tool, and
CVE-2023-38546, a low severity bug that only affects libcurl.

Release announcement: curl/curl#12026

Changes required:
bazel: upgrade version here -

opentelemetry-cpp/bazel/repository.bzl

Line 151 in 18a27df

"https://curl.haxx.se/download/curl-7.73.0.tar.gz",

For CMake, we don't use sticky version for libcurl, and rely on package manager (apt-get, vcpkg) so hopefully no changes be required.

Thanks @ThomsonTan for bringing this up. Please add if I missed something.