ci: bump go-version to 1.25 in CI + release workflows

go.mod requires go 1.25.0; both workflows were still pinned at 1.23. The
v0.6.0 release run failed at goreleaser's `go mod tidy` hook with
`go.mod requires go >= 1.25.0 (running go 1.23.12; GOTOOLCHAIN=local)` —
goreleaser sets GOTOOLCHAIN=local by default, which blocks Go's automatic
toolchain fetch, so setup-go must install the right version directly.

Bumps:
- .github/workflows/release.yml setup-go go-version "1.23" -> "1.25"
- .github/workflows/ci.yml setup-go go-version '1.23' -> '1.25' (both
  test and build matrix)

The v0.6.0 tag will be re-cut against this commit so the release
workflow runs the right toolchain.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: nachiket

.github/workflows/ci.yml

@@ -20,7 +20,7 @@ jobs:
       - uses: actions/checkout@v6
       - uses: actions/setup-go@v6
         with:
-          go-version: '1.23'
+          go-version: '1.25'
           cache: true
       - run: go vet ./...
       - run: go test -race ./...
@@ -41,7 +41,7 @@ jobs:
       - uses: actions/checkout@v6
       - uses: actions/setup-go@v6
         with:
-          go-version: '1.23'
+          go-version: '1.25'
           cache: true
       - name: build
         env:

.github/workflows/release.yml

@@ -16,7 +16,7 @@ jobs:
           fetch-depth: 0
       - uses: actions/setup-go@v6
         with:
-          go-version: "1.23"
+          go-version: "1.25"
           cache: true
 
       # Catch the easy mistake of pushing a tag without bumping VERSION.