Bump version to 1.3.8

[drinkbeer]

Bumps response_bank from 1.3.7 to 1.3.8.

What's new since 1.3.7

• Brotli splice cache slot handling — reserve fixed-length slots in a Brotli-compressed cached response body and splice per-request values into them at serve time, without recompressing the body (Add Brotli splice cache slot handling #103).
• Adds the brotli_splice dependency that powers it (Add brotli_splice gem as dependency #102).

Changes in this PR

• lib/response_bank/version.rb — VERSION 1.3.7 → 1.3.8
• Gemfile.lock — response_bank self-reference 1.3.7 → 1.3.8
• CHANGELOG.md — new file (Keep a Changelog format) with the 1.3.8 entry

Why

Cutting 1.3.8 so Storefront Renderer can depend on a tagged release of the Brotli splice feature (shop/world PR #910569) instead of pinning a branch SHA.

Intended follow-up after merge: tag v1.3.8 on main.

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