refactor(build): split build cache into its own settings script

[jamesarich]

What

Adopt meshtastic-android's build cache implementation by moving the cache config out of develocity.settings.gradle into a dedicated gradle/build-cache.settings.gradle.

Why

Keeps caching concerns separate from build-scan concerns and switches to the standard Gradle cache API, so the build cache no longer depends on the Develocity plugin to function. Mirrors the structure used in Meshtastic-Android.

Changes

• New gradle/build-cache.settings.gradle — uses the standard top-level buildCache {} API (instead of develocity { buildCache {} }); keeps the getMeshProperty env → local.properties → config.properties walk-up and the fork-safe push gating; uses logger.lifecycle instead of println.
• gradle/develocity.settings.gradle — stripped the buildCache {} block and the now-unused getMeshProperty helper; now only configures develocity { buildScan {} }.
• settings.gradle.kts — applies both scripts (build cache, then build scans).

Deviation from the source (intentional)

Meshtastic-android applies this file from both its root and build-logic/settings.gradle.kts, so it carries an isLogic check to label the log line. MQTTastic has no build-logic included build, so that branch would be dead code here — dropped it and personalized the log to MQTTastic Build: ....

Verification

• ./gradlew help evaluates settings cleanly (exit 0), confirming the top-level buildCache {} block resolves against the Settings object.
• With GRADLE_CACHE_URL set, the remote-cache branch evaluates without error and logs MQTTastic Build: Remote cache URL found.
• None of the three files are in spotless's scope, so no formatting/license-header impact.

Note: org.gradle.caching=true is already set in gradle.properties, so local caching is live now and remote activates once GRADLE_CACHE_URL is provided.

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