ci: run build, test and lint on Crystal 1.21.0 only (#2463)

The build-crystal and tests matrices fanned out over 1.19.0, 1.20.0 and
1.21.0, and lint pinned 1.20.0. Six jobs per run for versions we no
longer target, while the Dockerfile has already moved to
crystal:1.21.0-alpine — so the images CI exercised did not match what
ships.

Collapse both matrices to 1.21.0 and move lint onto the same version.
The matrix form is kept so extra versions can be re-added by editing a
single list. AGENTS.md still documented 1.19.0 as the CI and Docker
version; update it to match the tree.

Note that shard.yml keeps `crystal: ~> 1.19` — the declared support
range for consumers is unchanged, only what CI verifies narrows.

Author: hahwul

.github/workflows/ci.yml

@@ -27,7 +27,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        crystal-version: [1.19.0, 1.20.0, 1.21.0]
+        crystal-version: [1.21.0]
     steps:
       - uses: actions/checkout@v7
       - uses: crystal-lang/install-crystal@v1
@@ -103,7 +103,7 @@ jobs:
       - uses: actions/checkout@v7
       - uses: crystal-lang/install-crystal@v1
         with:
-          crystal: 1.20.0
+          crystal: 1.21.0
       - name: Install dependencies
         run: shards install
       - name: Run Ameba linter
@@ -118,7 +118,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        crystal-version: [1.19.0, 1.20.0, 1.21.0]
+        crystal-version: [1.21.0]
     steps:
       - uses: actions/checkout@v7
       - uses: crystal-lang/install-crystal@v1

AGENTS.md

@@ -17,7 +17,7 @@ Crystal-based attack surface detector that identifies endpoints by static analys
 
 ```bash
 # Docker build (for CI or consistent environments)
-docker run --rm -v $(pwd):/app -w /app crystallang/crystal:1.19.0-alpine sh -c "apk add --no-cache yaml-dev zstd-dev && shards install && shards build"
+docker run --rm -v $(pwd):/app -w /app crystallang/crystal:1.21.0-alpine sh -c "apk add --no-cache yaml-dev zstd-dev && shards install && shards build"
 
 # Local install (Ubuntu/Debian)
 curl -fsSL https://crystal-lang.org/install.sh | sudo bash
@@ -161,6 +161,6 @@ Key design notes:
 4. Verify basic functionality: `./bin/noir -b spec/functional_test/fixtures/crystal`
 
 ## Environment
-- Crystal ~> 1.19 (CI: 1.19.0)
-- Docker image: `crystallang/crystal:1.19.0-alpine`
+- Crystal ~> 1.19 (CI: 1.21.0)
+- Docker image: `crystallang/crystal:1.21.0-alpine`
 - Dependencies: `libyaml-dev`, `libzstd-dev`, `zlib1g-dev`, `pkg-config`