Bump go version to 1.20

[srikanthccv]

Since 1.17 is no longer supported and also helps in removing the hacky workaround in #203

[codecov]

Codecov Report

Patch coverage is 90.14% of modified lines.

Files Changed	Coverage
client/internal/packagessyncer.go	0.00%
client/types/callbacks.go	ø
server/httpconnection.go	0.00%
client/internal/mockserver.go	85.71%
server/serverimpl.go	94.73%
client/httpclient.go	100.00%
client/internal/clientcommon.go	100.00%
client/internal/httpsender.go	100.00%
client/internal/receivedprocessor.go	100.00%
server/callbacks.go	100.00%
... and 1 more

📢 Thoughts on this report? Let us know!.

[tigrannajaryan]

@srikanthccv the build failed after merging. Would you be able to take a look at it? https://github.com/open-telemetry/opamp-go/actions/runs/6226524274/job/16899600951

[srikanthccv]

Seems like flaky behaviour. I am looking into it.