chore(platform): PAYMENTS-11727 Release 0.8.3

[WillemHoman]

Finalizes the 0.8.3 release.

• Bump version.rb 0.8.2 → 0.8.3
• Move the per-Resque-job histograms entry (resque_job_queue_latency_seconds, resque_job_perform_duration_seconds) under a ## 0.8.3 changelog heading

Why: the v0.8.3 tag and GitHub release were cut on the PR #31 merge commit without bumping the version, so version.rb still read 0.8.2. The gemspec stamps the version from that constant, so nothing could ship as 0.8.3 (rubygems has nothing past 0.8.1).

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Note

Low Risk
Release metadata only (version constant and changelog); no runtime or metric behavior changes in this diff.

Overview
Aligns the published gem version with the 0.8.3 release after the tag was cut without a version bump.

lib/bigcommerce/prometheus/version.rb updates Bigcommerce::Prometheus::VERSION from 0.8.2 to 0.8.3 so the gemspec and RubyGems artifact match the intended release.

CHANGELOG.md replaces the ### Pending Release section with ## 0.8.3, keeping the existing note about opt-in Resque job histograms (resque_job_queue_latency_seconds, resque_job_perform_duration_seconds) under that version.

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