Fortio 1.63.4

Switched build to go1.21.8 (security fixes) and other dependency updates

What's Changed

• Bump google.golang.org/grpc from 1.61.0 to 1.61.1 by @dependabot in #889
• Bump fortio.org/scli from 1.13.1 to 1.14.1 by @dependabot in #888
• Bump google.golang.org/grpc from 1.61.1 to 1.62.0 by @dependabot in #891
• Bump golang.org/x/net from 0.21.0 to 0.22.0 by @dependabot in #894
• Bump docker/setup-buildx-action from 3.0.0 to 3.1.0 by @dependabot in #893
• Bump codecov/codecov-action from 4.0.1 to 4.0.2 by @dependabot in #892
• Bump github.com/golang/protobuf from 1.5.3 to 1.5.4 by @dependabot in #899
• Bump codecov/codecov-action from 4.0.2 to 4.1.0 by @dependabot in #897
• Bump google.golang.org/grpc from 1.62.0 to 1.62.1 by @dependabot in #898
• switch to go 1.21.8 and prep for 1.63.4 by @ldemailly in #896

Full Changelog: v1.63.3...v1.63.4