chore: update to ld-find-code-refs 2.18.3

[cspath1]

Picks up 2.18.3, which carries the pelletier/go-toml/v2 v2.4.3 bump (XRAY-1033007) on top of the toolchain pin and dependency work from 2.18.0. Scanner image ld-find-code-refs-github-action:2.18.3 is published on Docker Hub, and the 2.18.3 release binary was verified to report go1.26.8 with go-toml v2.4.3.

Also switches the usage examples from an exact Action tag to @v2. The 2.18.1 sync rewrote those lines to @v2.18.1, but this repo was tagged v2.18.0, so the examples have been pointing at a tag that does not exist and 404 when copy-pasted. Floating on @v2 fixes them and removes the need to rewrite them each release -- moving the v2 tag is what delivers a new scanner to consumers regardless. The prose in this README already described the root Action as @v2.

The dockerImage input keeps an exact tag; there is no floating :2 tag on Docker Hub for the scanner image.

Synced from ld-find-code-refs build/metadata/github-actions.

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Note

Overview
Bumps the underlying scanner container from 2.18.1 to 2.18.3 in the root Dockerfile and in the docker composite action’s default dockerImage (plus README mirror example).

README workflow samples now reference launchdarkly/find-code-references@v2 and …/docker@v2 instead of @v2.18.1, so copy-pasted workflows no longer point at a non-existent Action tag; mirrored-registry examples still use an explicit 2.18.3 image tag.

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