Remove lockfiles, manage Gradle dependencies with Dependabot (#1535)

Dependabot currently does not update lockfiles in multi-module projects
(see https://github.com/dependabot/dependabot-core/issues/14633)

To work around this issue, we will simply remove our lockfiles, and
change our version catalog to use fully specified versions.
The removal of lockfiles introduces two issues:

1. It is less visible what our dependency graph is
2. Our builds are potentially non-reproducible

To work around this, two mitigations are in place:

1. Enable `failOnDynamicVersions()`, which causes Gradle to fail the
build if any dependencies declare a version range
2. Enable GitHub dependency submission, which provides insight into the
project SBOM

pkl-bom/gradle.lockfile

@@ -1,4 +0,0 @@
-# This is a Gradle generated file for dependency locking.
-# Manual edits can break the build and are not advised.
-# This file is expected to be part of source control.
-empty=classpath