Simplify Python dependency installation in Vercel

[taesungh]

Previously, in order to use python3-saml with Python Serverless Functions on Vercel, we needed to manually install and provide system-level dependencies for headers needed by xmlsec via apps/site/vercel-lib.sh.

As of today, it appears providing these extra headers is no longer necessary due to one of many updates in the last few months. Notably, in March 2024, Vercel announced that Node.js v20 is fully supported for Builds and Vercel Functions, and with this newer version, the Build image that Vercel uses was upgraded to Amazon Linux 2023 which uses a newer version of glibc. This also means the AWS base image for Lambda functions running Python 3.12 also use AL2023. Besides that, there were newer releases of xmlsec/python-xmlsec v1.3.14 in April 2024 as well as lxml-5.3.0 in August.

I was unable to pinpoint which exact change alleviated the issue, but going forward we should be able to use the new build image and dependencies to simplify the process of building the project.

Vercel now uses Python 3.12 as the default Python version when using Node.js 20.x and up (Python 3.12 available in May 2024), so it would be good to also coordinate the changes with the version upgrade. (In practice, the API functionality is not being actively used right now, so the actual impact of breaking something should be minimal.)