Update README and RELEASE for PG18 1.8.0 release

[jrgemignani]

modified: README.md
modified: RELEASE

[Copilot]

Pull request overview

Updates release documentation for the Apache AGE 1.8.0 PG18 branch and refreshes README installation guidance around supported PostgreSQL versions.

Changes:

• Adjust RELEASE header to explicitly refer to the PG18 branch and add initial release-note entries for the 1.8.0 advancement/merge.
• Update README PostgreSQL compatibility statements to include PostgreSQL 19beta1 in the supported list.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
RELEASE	Clarifies the 1.8.0 release-notes header for the PG18 branch and adds top-of-list release-note entries.
README.md	Updates installation text describing which PostgreSQL versions are supported.

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[gregfelice]

LGTM — trivial README/RELEASE update for the PG18 1.8.0 release. CI green.

[gregfelice]

LGTM — co-approving.

Docs-only change (README.md + RELEASE, 5 additions / 3 deletions, no code):

• RELEASE: header retitled for master branch (currently PG18) → for PG18 branch (correct — master has advanced to PG19, PG18 is now its own branch), and two changelog lines prepended for the 1.8.0 merge into PG18 (Advance master branch to Apache AGE version 1.8.0 (#2455)), matching the merged #2466.
• README.md: supported-version list updated to add 19beta1, plus a stray <a> tag and a Postgres→PostgreSQL wording fix.

CI green across all build jobs.

Minor observation (non-blocking): on the PG18 branch the README now lists 19beta1 as supported — reads as the shared project-wide version list (consistent with #2463 / master) rather than a PG18 build claim, so fine as-is.