revert: move Request.Body streaming breaking changes off master, they belong to 14.x

[velo]

What

Reverts the Request.Body streaming breaking changes from master (the 13.x line):

• reverts fix: prevent Content-Length smuggling, log format-string injection, and common-pool starvation #3395 (f9159cfa — feat!: make feign.Request.Body streaming-ready): restores Request.create(HttpMethod, String, Map, byte[], Charset), Request.body() returning byte[], RequestTemplate#body(String)/body(byte[], Charset), removes MIGRATION-v14.md and the japicmp excludes that bypassed the binary-compatibility gate.
• reverts the API-adaptation test changes from Fix feign-core test compilation broken by removed Request.create overload #3406 (which only existed to chase that API).

What is kept

• The hermetic Http2ClientTest rewrite from Fix feign-core test compilation broken by removed Request.create overload #3406 (API-independent): tests no longer call the live https://nghttp2.org/httpbin/ service, which was silently hanging pr-build/snapshot for 10+ minutes whenever that service degraded. MockWebServer rejects GET-with-body, so that one test uses a minimal local socket server.
• [issue #2998] Expose uriTemplate in RequestTemplate #3393 (requestUriTemplate()) — additive, stays on master.

Why

#3395 was meant for the 14.x branch but was opened against master (the default base). 14.x already contains the streaming work via the last master→14.x merge, so nothing needs porting.

⚠️ Note for the next master→14.x merge

This revert commit must NOT be applied to 14.x — a plain merge would undo the streaming work there. Revert this revert in the merge commit, or resolve conflicts toward the 14.x side for the affected files.

Verified locally with a full mvn verify (all 55 modules green).

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