deps(core): bump reqwest to 0.13

[Xuanwo]

Which issue does this PR close?

N/A.

Rationale for this change

OpenDAL still depends on reqwest 0.12.x, while the current reqwest 0.13.x line changes the rustls feature name and simplifies the TLS defaults. This updates the core dependency chain to reqwest 0.13.2 and removes the tracing dev-dependency path that was still pulling reqwest 0.12 via opentelemetry-otlp defaults.

What changes are included in this PR?

This PR bumps reqwest to 0.13.2 in opendal-core and the top-level core crate, updates the internal reqwest-rustls-tls feature mapping to reqwest/rustls, disables default features for the tracing layer's opentelemetry-otlp dev-dependency so grpc-tonic does not reintroduce reqwest 0.12, and refreshes the reqwest docs links in the HTTP optimization guide.

Are there any user-facing changes?

Users building against OpenDAL's reqwest-backed HTTP client will now resolve reqwest 0.13.x instead of 0.12.x. The public API and documented dns_resolver(...) usage remain unchanged.

AI Usage Statement

AI-assisted editing and review were used while preparing this PR.