[Lazarus revival] Add deprecation warning to "Referrer" special-casing

[kax168]

Maintainer Briefing: Add deprecation warning to "Referrer" special-casing

What this PR was trying to do

Add an Express deprecation warning when req.get() relies on the historical Referer/Referrer special-casing, as discussed in issue expressjs#3951.

What changed in the repo since it died

The repository has changed substantially since PR expressjs#4106. Upstream now already includes depd and uses node:net; the revival reconciles those differences and merges cleanly with current origin/master. No listed intervening commit directly supersedes this deprecation.

What Lazarus changed to revive it

Rebased the original work onto current origin/master, preserved the deprecation behavior and original authorship, flattened an obsolete historical merge, and omitted the redundant dependency commit because depd is already present upstream. LAZARUS_TASK.md remains untracked and untouched.

Test evidence

Evidence	Before	After
Tests detected	Yes	Yes
Test command	npm test	npm test
Tests passed	No	Yes
Merge clean	Yes	Yes

Risk assessment

Risk appears low and localized to request-header lookup behavior. npm run lint passed, and post-revival npm test passed with 1,260 tests. The baseline run had 1,233 passing and one socket hang up failure in an unrelated res.format() negotiation test; no further evidence about that transient failure is available.

Credits

Originally contributed by @rileyjshaw.

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Original PR: expressjs#4106
Original author: @rileyjshaw