Fix serve certificate timezone issue

[TheEdda]

Hi, thanks for publishing this project. I love doing things not in Go.

I ran into an issue with the certificate validation failing (output from Terraform CLI log):

2025-09-16T18:08:22.729+0200 [DEBUG] provider.terraform-provider-test: WARNING: All log messages before absl::InitializeLog() is called are written to STDERR
2025-09-16T18:08:22.729+0200 [DEBUG] provider.terraform-provider-test: I0000 00:00:1758038902.729566 4499024 ssl_transport_security.cc:1884] Handshake failed with error SSL_ERROR_SSL: error:10000412:SSL routines:OPENSSL_internal:SSLV3_ALERT_BAD_CERTIFICATE
2025-09-16T18:08:22.729+0200 [WARN]  provider.stdio: stdio service not available, stdout/stderr syncing unavailable
2025-09-16T18:08:22.729+0200 [TRACE] GRPCProvider.v6: GetProviderSchema
2025-09-16T18:08:22.729+0200 [WARN]  provider.stdio: stdio service unavailable, run will do nothing
2025-09-16T18:08:22.729+0200 [ERROR] plugin6.(*GRPCProvider).GetProviderSchema: error="rpc error: code = Unavailable desc = connection error: desc = \"transport: authentication handshake failed: tls: failed to verify certificate: x509: certificate has expired or is not yet valid: current time 2025-09-16T18:08:22+02:00 is before 2025-09-16T18:08:21Z\""
2025-09-16T18:08:22.729+0200 [TRACE] GRPCProvider.v6: Close

Turns out the self-signed certificate generated by the server is not yet valid if your system timezone has a positive offset from UTC because:

• datetime.now() generates a datetime in your local timezone and with tzinfo=None
• x509.CertificateBuilder().not_valid_before() expects a datetime in UTC and treats tzinfo=None as UTC (source: https://cryptography.io/en/43.0.3/x509/reference/#cryptography.x509.CertificateBuilder.not_valid_before)

So I went ahead and fixed that :)

[hfern]

@TheEdward162 thank you for your contribution!