Co-authored-by: release-please[bot] <55107282+release-please[bot]@users.noreply.github.com>

…4632)

* fix: stop publishing a bigtable-beam-import pom with no dependencies

bigtable-beam-import attaches its shaded jar under a classifier, so the
main artifact is the thin, unshaded jar and its pom must keep every
compile dependency. maven-shade-plugin still strips them from the
generated dependency-reduced pom in that configuration (MSHADE-419),
which is not fixed in 3.6.0 despite what the comment in the root pom
claimed. The bump to 3.6.0 in #4626 therefore published 2.20.0 and
2.20.1 poms declaring zero compile dependencies.

Consumers resolving the artifact from Maven Central get no transitive
deps at all. GoogleCloudPlatform/DataflowTemplates hits this staging the
Cloud_Bigtable_to_GCS_SequenceFile template, which fails with
ClassNotFoundException: org.apache.hadoop.hbase.io.ImmutableBytesWritable.
Nothing caught it in-repo because the reactor resolves sibling modules
from source poms and never reads dependency-reduced-pom.xml.

- set createDependencyReducedPom=false explicitly on the module rather
  than relying on a plugin default that changed between 3.2.4 and 3.6.0
- add a verify-published-pom-deps goal to bigtable-build-helper that
  inspects the pom that would actually be deployed and fails the build
  if a required compile dependency is missing, wired into the module's
  verify phase; covered by integration tests for both the healthy and
  the MSHADE-419-stripped case
- correct the root pom comment and drop the renovate rule that pinned
  maven-shade-plugin away from 3.3.0, which was both an incomplete
  description of the bug and moot now that the flag is explicit

Change-Id: I2d269c60175e10da7e5b361f410dd06f7861ba48

* fix: guard against a missing or half-declared pom in verify-published-pom-deps

Change-Id: Ice5b8a44978b4833bc48c5fe037a263bbce68b77

Co-authored-by: release-please[bot] <55107282+release-please[bot]@users.noreply.github.com>