Update deps for HTTP/2 rapid reset

fixes CVE-2023-44487 by updating deps

Go mod update (1.18)

Update to go 1.18, and update all deps.

Go mod update

go mod update again. Also minor logging text update.

Go mod update (1.17)

Updated deps and moved to go 1.17. No code changes.

Go mod update

update go version to 1.16, and updated deps.

Underlying API update

• It looks like Google released a stable API version for the Secret Manager API, though there were no changes that impacted this library. I went ahead and moved to it. This moves from the v1beta1 API to the v1 api for Secret Manager.
• Google updated the docs to say secret names are limited to 255 chars, which I've documented and now trim secret names to that length.
• Did some minor lint cleanup.

Readme updates

smcache should be ready for use by others. It has been the backing store for one of my website for ~5 months, and has gone through 1 cert rotation without issue. Readme's are now up-to-date and should give a good starting place for others that want to use this library.

I'll likely promote this to the 1.0.0 release once doing one more review of the code.

Tests complete

• Near 100% test coverage now.
• Cleaned up some lint warnings, nothing major.
• Included a couple examples in the repo, which gives nice sanity checks on 'go build', plus shows how to use it.