ci: Remove Codecov and code coverage tooling (JAVA-560) (#5547)

* ci: Remove Codecov and code coverage tooling (JAVA-560)

Remove the Codecov service integration (codecov.yml, the README badge, and the upload steps across all CI workflows) along with the JaCoCo and Kover coverage tooling that only existed to feed it: the plugins, report and verification tasks across all modules, the version catalog entries, the Config.kt coverage threshold, and the createCoverageReports Makefile target.

No SDK code or public API is affected.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

* test(sentry): Restore java.lang open after jacoco removal (JAVA-560)

SentryTest reflectively rewrites a class's name to fake the Android environment, which requires --add-opens java.base/java.lang=ALL-UNNAMED. The jacoco test agent was implicitly providing this open; now that jacoco is removed, declare it explicitly so the sentry unit tests keep passing.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

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Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>