[build] update go version to 1.18

[pgier]

Motivation

Upgrade module golang version from 1.15 to 1.18. Golang 1.18 is the oldest currently supported version. See: https://go.dev/doc/devel/release. Client applications using go 1.15 will still be able to compile against this module, but we should recommend that they upgrade their golang version or stick with a previous release of pulsar-client-go, e.g. 0.9.0.

Additional motivation for this is that I'd like to upgrade most/all of the dependencies in a separate commit, and some of them are likely not fully compatible with golang 1.15

Modifications

Updated the golang version in go.mod and ran go mod tidy.

Verifying this change

• Make sure that the change passes the CI checks.

(Please pick either of the following options)

This change is covered by existing CI which runs the build using various golang versions.

Does this pull request potentially affect one of the following parts:

If yes was chosen, please highlight the changes

• Dependencies (does it add or upgrade a dependency): (no)
• The public API: (no)
• The schema: (no)
• The default values of configurations: (no)
• The wire protocol: (no)

Documentation

• Does this pull request introduce a new feature? (no)
• If yes, how is the feature documented? (not applicable)
• If a feature is not applicable for documentation, explain why? Added a minor doc update to the README
• If a feature is not documented yet in this PR, please create a followup issue for adding the documentation

[zzzming]

LGTM

[tisonkun]

Client applications using go 1.15 will still be able to compile against this module

@pgier I'm curious about this statement. If we follow write some code depending on new added APIs, how can a lib compile in 1.15 compile with us as a dependency? Said os.ReadFile.

[pgier]

@tisonkun You are right at that point we will no longer be compatible with go 1.15. We should see a failure in the 1.15 integration tests at the point when a PR includes an incompatible change, and we would have to either not accept the change, or remove the 1.15 tests. My point was just that this change by itself doesn't make us incompatible with 1.15.

I'll work on upgrading dependencies next and see if anything fails, and we can decide whether it's worth upgrading specific incompatible deps.

[tisonkun]

@pgier Thanks for your explanation! I see the IT cases whose matrix includes 1.15 now.

[tisonkun]

Good to go. One comment inline.