chore: update to go1.20.8

[bassosimone]

This pull request updates ooni/oohttp to use go1.20.8.

This is the update check list:

• update UPSTREAM, commit the change, and then
  run the ./tools/merge.bash script to merge from upstream;
• make sure you synch ./internal/safefilepath with the
  ./src/internal/safefilepath of the Go release you're merging from;
• solve the very-likely merge conflicts and ensure the original spirit of the
  patches still hold;
• make sure the codebase does not assume *tls.Conn anywhere (git grep -n '\*tls\.Conn')
  and otherwise replace *tls.Conn with TLSConn;
• make sure the codebase does not call tls.Client anywhere (git grep -n 'tls\.Client')
  and otherwise replace tls.Client with TLSClientFactory;
• ensure go build -v ./... still works;
• ensure go test -race ./... is still passing;
• ensure stdlibwrapper.go copies all
  the Request and Response fields;
• run go get -u -v ./... && go mod tidy;
• commit the changes and push merged-main to gitub;
• open a PR and merge it using a merge commit;
• create a new working branch to update the examples;
• ensure example/internal/utlsx/utlsx.go
  copies all the ConnectionState fields;
• go to example, update each submodule and ensure
  go test -race ./... passes in each submodule;
• open a PR and merge it using a merge commit.

[bassosimone]

About the safefilepath package:

% diff -ru . ~/src/github.com/ooni/oohttp/internal/safefilepath
diff --color -ru ./path_test.go $HOME/src/github.com/ooni/oohttp/internal/safefilepath/path_test.go
--- ./path_test.go	2023-10-04 14:04:25
+++ $HOME/src/github.com/ooni/oohttp/internal/safefilepath/path_test.go	2023-10-04 14:10:07
@@ -5,11 +5,12 @@
 package safefilepath_test
 
 import (
-	"internal/safefilepath"
 	"os"
 	"path/filepath"
 	"runtime"
 	"testing"
+
+	"github.com/ooni/oohttp/internal/safefilepath"
 )
 
 type PathTest struct {

% pwd
$HOME/src/github.com/golang/go/src/internal/safefilepath

% git rev-parse HEAD
d5b851804329aa547dafa278a0c35dd62298d651