Update Go toolchain to 1.20.5

Otherwise building Gazelle fails with
```
ERROR: loading failure: Traceback (most recent call last):
        File "/home/runner/.cache/bazel-repo/bazel/_bazel_runner/e97befc866f153aa83e528704066dbd2/external/bazel_gazelle/internal/go_repository_tools.bzl", line 103, column 13,
 in _go_repository_tools_impl
                fail("failed to build tools: " + result.stderr)
Error in fail: failed to build tools: # github.com/bazelbuild/bazel-gazelle/cmd/fetch_repo
src/github.com/bazelbuild/bazel-gazelle/cmd/fetch_repo/go_mod_download.go:64:17: undefined: errors.Join
ERROR: @@com_github_pmezard_go_difflib//difflib:difflib: analysis failure: no such package '@@com_github_pmezard_go_difflib//difflib': no such package '@@bazel_gazelle_go_repos
itory_config//': no such package '@@bazel_gazelle_go_repository_tools//': failed to build tools: # github.com/bazelbuild/bazel-gazelle/cmd/fetch_repo
src/github.com/bazelbuild/bazel-gazelle/cmd/fetch_repo/go_mod_download.go:64:17: undefined: errors.Join
ERROR: //util:update: analysis failure
```
See https://github.com/aherrmann/rules_zig/actions/runs/9230203968/job/25413273025?pr=312#step:9:87

WORKSPACE

@@ -94,7 +94,7 @@ load("@io_bazel_rules_go//go:deps.bzl", "go_register_toolchains", "go_rules_depe
 
 go_rules_dependencies()
 
-go_register_toolchains(version = "1.19.3")
+go_register_toolchains(version = "1.20.5")
 
 gazelle_dependencies()