I figured out the solution. According to this URL:
https://docs.gitlab.com/ee/administration/troubleshooting/ssl.html#using-an-internal-ca-certificate-with-gitlab
Under the Unable to perform Git operations due to an internal or self-signed certificate section
It mentions the git config --global http.sslCAInfo configuration.
So I did a git config --list and found the location of our ca-bundle.crt file, which also had expired just last week.
Not only had our GitLab cert expired, but so did the CA Bundle (certificate chain).
The next issue was, I had issues exporting the Windows CA Certificate Chain and converting the P7B to CRT using OpenSSL.
I decided to export the Sub CA certificate as a CER, and then converted it from CER to CRT (OpenSSL).
I next exported the Root CA certificate as a CER, and also converted it from CER to CRT (OpenSSL).
Following instructions from this URL, I built my own bundle using the CRT files:
https://ssl4less.eu/faq/technical-questions/how-do-i-make-my-own-bundle-file-from-crt-files.html
So basically, I opened up both crt files within a text editor (Notepad++), copied the RootCA cert text, and pasted it into the IssuingCA cert at the very bottom and then saved the file. Worked like a charm.
Now Jenkins is properly working with GitLab again. What a crazy few days.