Update dotnet-grpc tool to require .NET 6

[JamesNK]

There is a package conflict warning that I can't figure out in netcoreapp3.1. Updating the command line tool to net6.0 fixes it.

Note that .NET Core 3.1 hits end-of-life in December. See https://dotnet.microsoft.com/en-us/platform/support/policy/dotnet-core

[JamesNK]

@jtattermusch Nuget Build step is intermittently failing with this error:

Warning: Permanently added 'localhost' (ED25519) to the list of known hosts.
Warning: Permanently added 'localhost' (ED25519) to the list of known hosts.
Warning: Permanently added 'localhost' (ED25519) to the list of known hosts.
[01:41:34] Transferring environment variable script to build VM
[01:41:35] Transferring kokoro_log_reader.py to build VM
[01:41:36] Transferring source code to build VM
[01:41:37] Executing build script on build VM



[ID: 3662310] Executing command via SSH:
export KOKORO_BUILD_NUMBER="2357"
export KOKORO_JOB_NAME="grpc/dotnet/presubmit/build_nuget"
source /tmpfs/kokoro-env_vars.sh; cd /tmpfs/src/ ;  chmod 755 github/grpc-dotnet/kokoro/build_nuget.sh ;  PYTHON_3_VERSION="$(pyenv which python3 2> /dev/null || which python3)" ; PYTHON_2_VERSION="$(pyenv which python2 2> /dev/null || which python2)" ; if "$PYTHON_3_VERSION" -c "import psutil" ; then KOKORO_PYTHON_COMMAND="$PYTHON_3_VERSION" ; else KOKORO_PYTHON_COMMAND="$PYTHON_2_VERSION" ; fi > /dev/null 2>&1 ; echo "export KOKORO_PYTHON_COMMAND="$KOKORO_PYTHON_COMMAND"" > "$HOME/.kokoro_python_vars" ; nohup bash -c "( rm -f /tmpfs/kokoro_build_exit_code ; github/grpc-dotnet/kokoro/build_nuget.sh   ; echo \${PIPESTATUS[0]} > /tmpfs/kokoro_build_exit_code ) > /tmpfs/kokoro_build.log 2>&1" > /dev/null 2>&1 & echo $! > /tmpfs/kokoro_build.pid ; source "$HOME/.kokoro_python_vars" ; "$KOKORO_PYTHON_COMMAND" /tmpfs/kokoro_log_reader.py /tmpfs/kokoro_build.log /tmpfs/kokoro_build_exit_code /tmpfs/kokoro_build.pid /tmpfs/kokoro_log_reader.pid --start_byte 0

++ dirname github/grpc-dotnet/kokoro/build_nuget.sh
+ cd github/grpc-dotnet/kokoro/..
+ export DOTNET_SKIP_FIRST_TIME_EXPERIENCE=true
+ DOTNET_SKIP_FIRST_TIME_EXPERIENCE=true
+ sudo apt-get install -y jq
Reading package lists...
Building dependency tree...
Reading state information...

No apt package "jq", but there is a snap with that name.
Try "snap install jq"

E: Unable to locate package jq


[ID: 3662310] Command finished after 1 secs, exit value: 100


Warning: Permanently added 'localhost' (ED25519) to the list of known hosts.
[01:41:39] Collecting build artifacts from build VM
Build script failed with exit code: 100

[BrennanConroy]

return SHA256.HashData(stream);

[JamesNK]

Good idea, but SHA256.HashData doesn't support stream: https://learn.microsoft.com/en-us/dotnet/api/system.security.cryptography.sha256.hashdata?view=net-6.0

[BrennanConroy]

Oh sad, it's 7.0 https://apisof.net/catalog/e98725f1-0fcc-2eca-c31d-23e541ddcef8

[jtattermusch]

LGTM. I also restarted the Nuget build as there seem to have been an infrastructure flake (which should now be resolved).