.github: Use golangci-lint v2.6.0 in our PR checks

This is just a routine upgrade, keeping us up-to-date with the latest
linter changes. This already passes when run on Linux.

We know that currently the lint checks aren't passing on Windows even with
v2.4.0, and this doesn't do anything about that but it does mean that once
we make the linters pass on Windows we'll be dealing with the full set
supported in v2.6.0, rather than potentially having to do this across
two commits.

Signed-off-by: Martin Atkins <mart@degeneration.co.uk>

.github/workflows/checks.yml

@@ -171,7 +171,7 @@ jobs:
       - name: "Code linting"
         uses: golangci/golangci-lint-action@4afd733a84b1f43292c63897423277bb7f4313a9 # v8.0.0
         with:
-          version: v2.4.0
+          version: v2.6.0
       
       - name: "Copyright headers"
         run: |

Makefile

@@ -47,7 +47,7 @@ protobuf:
 # Golangci-lint
 .PHONY: golangci-lint
 golangci-lint:
-	go run github.com/golangci/golangci-lint/v2/cmd/golangci-lint@v2.4.0 run --timeout 60m ./...
+	go run github.com/golangci/golangci-lint/v2/cmd/golangci-lint@v2.6.0 run --timeout 60m ./...
 
 # Run license check
 .PHONY: license-check