chore(fireflyiii_fints_importer): correct updater.json slug, drop unused build arg

updater.json claimed `slug: fireflyiii_data_importer`, which is a different
add-on in this repo. The field is metadata only — addons_updater derives the
slug from the directory name (`SLUG=${f//\//}` in
addons_updater/rootfs/etc/cont-init.d/99-run.sh) and never reads `.slug` from
updater.json — so this corrects misleading bookkeeping rather than changing
behaviour.

The same file quoted two booleans as strings. `jq -r` yields the identical
text `true` for `true` and for `"true"`, so the two forms are equivalent to
every reader in the updater script; unquoting them matches how every other
updater.json in the repo spells these flags.

`ARG BUILD_UPSTREAM="1.2"` was both stale (upstream is 1.3.0) and unused:
nothing in the Dockerfile references it and the builder does not pass a
matching --build-arg. This add-on takes the application from a prebuilt
upstream image, so there is no upstream version to record at build time;
removing the line is more accurate than updating the number.

Left alone deliberately: `upstream_version` and `paused`. With
`dockerhub_by_date: true`, unpausing would make addons_updater compare
`1.3.0` against the newest Docker Hub date tag and rewrite config.yaml's
version accordingly — that is the updater's decision to make, not this PR's.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

fireflyiii_fints_importer/Dockerfile

@@ -16,7 +16,6 @@
 
 ARG BUILD_FROM
 ARG BUILD_VERSION
-ARG BUILD_UPSTREAM="1.2"
 FROM ${BUILD_FROM}
 
 ##################

fireflyiii_fints_importer/updater.json

@@ -1,10 +1,10 @@
 {
-  "dockerhub_by_date": "true",
-  "github_fulltag": "true",
+  "dockerhub_by_date": true,
+  "github_fulltag": true,
   "last_update": "20-09-2025",
   "paused": true,
   "repository": "alexbelgium/hassio-addons",
-  "slug": "fireflyiii_data_importer",
+  "slug": "fireflyiii_fints_importer",
   "source": "dockerhub",
   "upstream_repo": "benkl/firefly-iii-fints-importer",
   "upstream_version": "1.3.0"