Use ECDSA(P-256) instead of RSA for Pages Let's Encrypt Integration

What does this MR do and why?

As per the benchmarking results in https://gitlab.com/gitlab-org/gitlab/-/issues/361168#note_933171008 , it was noticed that using ECDSA(P-256) is much more efficient than using RSA for generating Let's Encrypt Certificates for Pages and the resulting TLS handshake. It would result in performance boost and would help us in avoiding multiple production issues where the CPU is saturated while performing TLS Handshake.

Fixes https://gitlab.com/gitlab-org/gitlab/-/issues/361168

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