docs: drop stale Experimental banners, recommend PG18

[pyramation]

Summary

Docs-only (READMEs). Plan: constructive-io/constructive-planning#2208.

• plpgsql-parser and plpgsql-deparser: remove the "⚠️ Experimental: This package is currently experimental" banner. Both are production dependencies of constructive-db and constructive. The links to the related packages (pgsql-parser, pgsql-deparser, plpgsql-parser) stay, as plain notes.
• pgsql-deparser: "Latest Version (PostgreSQL 17)" becomes 18, and PG18 is now the recommendation. PG17 is still supported through @pg17, which is added to the version-specific install list. In the version table, PG14-16 changes from "⚠️ Experimental (hardening in progress)" to "Legacy", since those dist-tags (14.1.1 / 15.1.1 / 16.1.1) aren't being developed.
• @pgsql/transform-ast (README + transformers-direct/README.md): the docs still said PG17 was the target, but transformToLatest already goes to 18 and the PG*ToPG18Transformer / V17ToV18Transformer classes ship. Examples and tables now use the PG18 transformers, and the PG17 ones are still listed.

pgsql-types keeps its Experimental note on purpose. Nothing depends on it yet.

Link to Devin session: https://app.devin.ai/sessions/abe99c93b1e24e9c85346420f90d1f36
Open in Devin Desktop: https://app.devin.ai/desktop/session/abe99c93b1e24e9c85346420f90d1f36?variant=devin
Requested by: @pyramation

[devin-ai-integration]

I'll fix CI failures and address comments from users with write access. I'll skip comments containing "(aside)".

• Disable automatic comment, CI, and merge conflict monitoring

[tenki-reviewer]

Review complete. No issues found — approved ✅.

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Documentation-only change refreshing package READMEs for the PostgreSQL 18 release: it documents new pg18-era dist-tags, marks PG17 as fully supported, downgrades PG14-16 to legacy status, and describes the new direct transformers in transform-ast. All documented APIs, import paths, transformer classes, and version claims were verified against the actual source, config/versions.json, and shipped libpg-query bindings; no inaccuracies were found.

Files	Change
packages/deparser/README.md	Adds PG18 support status, pg18 dist-tag guidance, and updated version support table.
packages/plpgsql-deparser/README.md	Refreshes version-support wording for the PL/pgSQL deparser.
packages/plpgsql-parser/README.md	Refreshes version-support wording for the PL/pgSQL parser.
packages/transform-ast/README.md	Documents PG13-PG18 transformation matrix, direct transformers, and 18/types module paths.
packages/transform-ast/src/transformers-direct/README.md	New per-transformer docs for v13-to-v18 through v17-to-v18 direct transformer classes.

_{Reviewed commit: 186d52c}