chore: upgrade to go1.21.9

[bassosimone]

Closes ooni/probe#2702

Closes ooni/probe#2695

[bassosimone]

I'm currently blocked by not being able to upgrade to the new version of Psiphon.

Here's what is happening:

% ./script/go.bash get -v github.com/Psiphon-Labs/psiphon-tunnel-core@71ac166e6917a8d758f2e09ca7f148f7c337a754
+ cd ./pkg/gobash
+ go build -v -o gobash.exe .
github.com/ooni/probe-cli/pkg/gobash
+ ./pkg/gobash/gobash.exe download
go1.21.9: already downloaded in /Users/sbs/sdk/go1.21.9
+ ./pkg/gobash/gobash.exe get -v github.com/Psiphon-Labs/psiphon-tunnel-core@71ac166e6917a8d758f2e09ca7f148f7c337a754
go: accepting indirect upgrade from github.com/Psiphon-Labs/quic-go@v0.0.0-20230626192210-73f29effc9da to v0.0.0-20240305203241-7c4a760d03cc
go: accepting indirect upgrade from github.com/refraction-networking/conjure@v0.7.10 to v0.7.11-0.20240130155008-c8df96195ab2
go: github.com/Psiphon-Labs/psiphon-tunnel-core@v1.0.11-0.20240405152454-71ac166e6917 requires
	github.com/Psiphon-Labs/quic-go@v0.0.0-20240305203241-7c4a760d03cc requires
	github.com/Psiphon-Labs/psiphon-tunnel-core@v2.0.28+incompatible, but v1.0.11-0.20240405152454-71ac166e6917 is requested
go: github.com/Psiphon-Labs/psiphon-tunnel-core@71ac166e6917a8d758f2e09ca7f148f7c337a754 (v1.0.11-0.20240405152454-71ac166e6917) indirectly requires github.com/Psiphon-Labs/psiphon-tunnel-core@v2.0.28+incompatible, not github.com/Psiphon-Labs/psiphon-tunnel-core@71ac166e6917a8d758f2e09ca7f148f7c337a754 (v1.0.11-0.20240405152454-71ac166e6917)

I have requested specific help to Psiphon developers to help me understand this, since I am a bit lost 🤔.