Drop Built-Date, Built-JDK, Created-By from plugin manifests

[Vest]

Summary

code/gradle/plugins.gradle:30 set three attributes on every plugin jar manifest. The first one, Built-Date: new Date(), was evaluated at configuration time on every Gradle invocation, with two consequences:

1. Non-reproducible builds. Two consecutive clean builds produced different bytes inside the manifest, so the resulting jar SHAs differed.
2. Configuration-cache miss. Reading the wall-clock during the configuration phase makes the plugin tasks incompatible with Gradle's configuration cache.

The companion Built-JDK and Created-By attributes are dropped at the same time because they're redundant: nothing in the codebase reads any of the three (grep -rn 'Built-Date|Built-JDK|Created-By' returns only the writer line), and the main pcgen.jar manifest carries no equivalents — the plugin jars were the odd ones out.

If we want truly reproducible builds, that's a larger, project-wide change: it would have to pin archive timestamps (preserveFileTimestamps = false), file ordering (reproducibleFileOrder = true), and the JDK/Gradle versions across environments, applied to all archive tasks. Keeping Built-JDK/Created-By on plugin jars only would be inconsistent — leaking environment info from a subset of artifacts without buying real reproducibility. A future PR pursuing full reproducible builds can apply those settings project-wide and reintroduce diagnostic attributes uniformly across every Java module.

Test plan

• ./gradlew :jarAllPlugins — all 11 plugin jars build successfully.
• Inspected plugins/exportplugins.jar manifest — Built-Date, Built-JDK, and Created-By removed; Implementation-Title, Implementation-Version, and Class-Path retained.
• Reproducibility check: two consecutive clean builds of :jarExportPlugins produce byte-identical jars (same SHA-1: `1b5ed939...`).
• Build output reports Configuration cache entry stored. for :jarAllPlugins.