fix(build): bump version to 0.20.3

[fupelaqu]

First of the six-repo release train: core → extensions → arrow → jdbc → repl → helm. Nothing downstream can move until this is merged and published.

What is in 0.20.3

Nine commits since 0.20.2, four of them R1 defect closures:

story	issue	change
R1FIX.3	#184	preserve the HTTP status of thrown ES client exceptions — None ≠ 404 ≠ 500, so IF EXISTS can tell absent from the cluster is down
R1FIX.4	#183	COPY INTO reads local files without Hadoop, plus Hadoop 3.4.3
R1FIX.5	#185	single-table materialized views are supported — one transform, one stage
R1FIX.8	—	a DDL statement can succeed with a warning: DdlResult(success: Boolean, warnings: Seq[String] = Nil)

plus install.ps1 parity, the logback appender fix and the doc version sweep already on main.

#183 stays open on purpose: the local-file fast path is fixed, but Parquet, Delta and remote sources are still Hadoop-bound and still break from JDK 23.

⚠️ Release notes — three user-visible changes

1. DdlResult gained a second field. Positional pattern matches (case DdlResult(ok)) need a trailing _; construction is unchanged thanks to the default. Rendering: ascii and json carry the warnings, csv does not — CsvFormatter has no DdlResult case at all and falls through to "No data". That predates R1FIX.8 but is newly consequential, since the whole point is that the warning reaches the user.
2. Honest materialized-view stage counting rejects the previously published REFRESH EVERY 8 SECONDS + delay = '2s' example, and the calculated delay moved from frequency/(1.5 × stages) to frequency/(2 × stages).
3. A cluster with xpack.watcher.enabled: false (HTTP 400, not 403) now hard-fails CREATE MATERIALIZED VIEW where it previously succeeded silently, having created a view whose watcher_id pointed at a watcher that did not exist.

Verification

sbt "show version" — the build loads and every module reports 0.20.3.

The behaviour behind these fixes is covered end-to-end, through packaged artefacts against a real Elasticsearch on a basic licence, by the three acceptance suites added this cycle: jdbc .github/scripts/acceptance.sh, repl .github/scripts/acceptance.sh (ES 6/7/8/9), and the arrow Flight SQL python suite.