docs: clarify proxy runtime history (#93467)
### What?
Clarifies the Proxy docs so the “Why Proxy” section no longer appears to
contradict the Runtime section.
### Why?
The Runtime section states that Proxy defaults to the Node.js runtime,
but the “Why Proxy” section still said Middleware defaults to the Edge
Runtime. Since Middleware was renamed to Proxy in v16, that wording
could make readers think the current default runtime is Edge.
### How?
Updates the “Why Proxy” paragraph to frame Edge Runtime wording as
historical context rather than current default behavior.
Also updates the v16.0.0 version history entry to mention that Proxy
defaults to the Node.js runtime.
Closes #93463
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Co-authored-by: Joseph <sephxd1234@gmail.com>
