docs: bring user-facing versions up to the 0.20.3 release train

[fupelaqu]

Post-release documentation sweep for the 0.20.3 train.

Version bookkeeping

Every version below was checked against JFrog before being written, not inferred from the train:

what	from	to
jdbc-driver / adbc-driver / arrow-flight-sql	0.2.3	0.2.4
community-extensions	0.2.2	0.2.3
arrow-extensions	0.2.3	0.2.4
java-client (engine)	0.20.2	0.20.3
installer examples + bundle-version list	0.20.1 / 0.20.2	0.20.3

The 0.20.2 sweep had to fix a doc that installed a version which never shipped, and the failure mode is quiet: a version with no -all bundle silently falls back to the plain artifact, which has no arrow extensions and therefore no cross-index JOIN. The docs would be demonstrating the path that loses the headline feature. So softclient4es{6,8,9}-cli-all_2.13:0.20.3, *-adbc-driver:0.2.4, *-arrow-flight-sql:0.2.4, *-jdbc-driver:0.2.4 and softclient4es8-java-client_2.13:0.20.3 were each confirmed 200 first.

Two corrections that are not bookkeeping

1. dml_statements.md told 0.20.3 users their own release was broken.

It said "releases up to and including 0.20.3 fail on JDK 23 and newer" for PARQUET, DELTA_LAKE and remote schemes — sitting directly beneath a table stating those work on every JDK. The boundary is 0.20.2, confirmed from the published POMs rather than the commit title:

core_2.13-0.20.3.pom   hadoop-client 3.4.3   ← HADOOP-19212, the fix
core_2.13-0.20.2.pom   hadoop-client 3.4.2

2. materialized_views.md quoted a watcher warning that is not the one that ships.

The doc's version named the licence as the cause. The shipped wording deliberately names no cause, because the licence is only one of two ways to reach it — the other is a cluster with no usable webhook credentials, which is exactly what xpack.security.enabled: false gives you (the default local, CI and testkit setup). The real cause is quoted verbatim after Reason:.

Replaced with the exact output captured from the -all bundle against a basic cluster, and documented the second cause — plus the boundary that keeps the degradation honest: a wrong credential still fails loudly, because that is a fixable typo, not a missing capability.

Left alone

Historical statements: since v0.14.0 feature markers and CONTRIBUTING's release-process example describe the past, not what to install.

ThisBuild / version stays at 0.20.3. This repo bumps to the next -SNAPSHOT with the first post-release change rather than in a dedicated commit — say the word if you would rather open 0.20.4-SNAPSHOT here.