Stop restricting TLS listeners to the P-256 key exchange group

[bardiharborow]

Remove the set_tmp_ecdh call in ServerContextFactory.configure_context. On OpenSSL 1.1.0+ it replaces the default list of supported groups with only P-256, so TLS listeners could not negotiate X25519 or X25519MLKEM768. The call was needed on OpenSSL 1.0.x, which enabled no curves by default. Synapse now requires cryptography>=3.4.7, so OpenSSL is always 1.1.0 or later. Twisted's CertificateOptions already skips this step on 1.1.0+.

Fixes #20373

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