databases/couchdb3: Resolve bundled OTP versions dynamically in PLIST_SUB

The release bundles erts and 13 OTP applications from
lang/erlang-runtime28, and pkg-plist pinned each of their versioned
directory names. Six of the last eight plist commits were pure OTP
rename churn, the largest touching 610 lines, while the 3.5.2 update
itself touched 23.

Derive the versions from the installed runtime at plist-generation time
instead. The package file list is unchanged: resolving the tokenized
plist with the versions this build staged reproduces the previous
pkg-plist byte for byte, so no PORTREVISION bump is needed.

An erlang-runtime28 update now needs only a PORTREVISION bump. Note
this reads ${LOCALBASE}/lib/erlang28 rather than the staged release,
because generate-plist runs before do-install; it holds as long as the
release copies erts and the OTP applications unmodified.

CouchDB's own rebar dependencies stay pinned deliberately - they move
only on a CouchDB update, where the change is worth seeing in review.

Approved by: dch (maintainer)
Differential Revision: https://reviews.freebsd.org/D59185

(cherry picked from commit aaa28e3db45298305baf558867d6d4ea43e2ac63)