chore: adapt to go 1.27

Restore the toolchain pin renovate dropped. The go directive has to lose
its patch version for that: go removes a toolchain equal to it, and every
go command then refuses to run until go.mod is tidied.

Go 1.27 changes the flate encoder, so the sizes and checksums pinned by
package tests move with the compressed output. Hash the avatar test input
directly instead of a png, which has no bearing on what it asserts.

encoding/json v2 is no longer an experiment, so drop the v1 implementation
and GOEXPERIMENT=jsonv2 with it.

Assisted-by: Claude Code:Opus 5

.github/workflows/cache-seeder.yml

@@ -37,7 +37,7 @@ jobs:
       - uses: ./.github/actions/go-setup
       - run: make deps-backend deps-tools
       - run: TAGS="bindata" make backend
-      - run: TAGS="bindata gogit" GOEXPERIMENT="" make backend
+      - run: TAGS="bindata gogit" make backend
       - name: warm test compile cache (bindata)
         env:
           TAGS: bindata
@@ -46,13 +46,12 @@ jobs:
       - name: warm test compile cache (bindata gogit)
         env:
           TAGS: bindata gogit
-          GOEXPERIMENT:
           GOTEST_FLAGS: -race -list=^$$ -count=1
         run: make test-backend
       - name: warm integration compile cache
         run: |
           TAGS="bindata" make test-integration-compile
-          TAGS="bindata gogit" GOEXPERIMENT="" make test-integration-compile
+          TAGS="bindata gogit" make test-integration-compile
           TAGS="bindata gogit" GOTEST_FLAGS="-race" make test-integration-compile
 
   lint:

.github/workflows/pull-db-tests.yml

@@ -96,7 +96,6 @@ jobs:
       - run: make backend
         env:
           TAGS: bindata gogit
-          GOEXPERIMENT:
       - run: GITEA_TEST_DATABASE=sqlite make test-migration
         env:
           TAGS: bindata gogit
@@ -107,7 +106,6 @@ jobs:
           # sqlite driver can contain large amount of Golang code, so don't use race detector for it, otherwise, extremely slow
           GOTEST_FLAGS: -timeout=40m
           TAGS: bindata gogit
-          GOEXPERIMENT:
 
   test-unit:
     if: needs.files-changed.outputs.backend == 'true'
@@ -170,7 +168,6 @@ jobs:
         env:
           GOTEST_FLAGS: -race -timeout=20m
           TAGS: bindata gogit
-          GOEXPERIMENT:
           GITHUB_READ_TOKEN: ${{ secrets.GITHUB_READ_TOKEN }}
           GITEA_TEST_CI_SKIP_EXTERNAL: true
       - run: make test-check