Add support for Spring Boot 2.x and Java 8

[adamlogic]

Note

Medium Risk
Adds a new distribution module and changes a core type (Metric record to class) for Java 8 compatibility, which can affect downstream binary/source compatibility and publishing outputs.

Overview
Adds legacy support for Spring Boot 2.x / Java 8 via a new judoscale-spring-boot-2-starter, including auto-configuration (filter + scheduled reporter), a Java 8-compatible HTTP client (Apache HttpClient), and generated adapter version metadata.

Updates the build to accommodate mixed Java targets (default Java 21, but judoscale-core and the new SB2 starter compile for Java 8), adds Spring Boot 2 dependency coordinates to libs.versions.toml, and wires in a new Spring Boot 2 sample app.

Refreshes docs and release automation to reference both starters and to auto-bump all README version snippets during release-please PRs.

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