Update the Client Libraries Issues link(s) in the README to point to template for filing an issue

[ohmayr]

For projects where we are creating issues directly in google-cloud-python we could update this link to point to template for filing an issue: For example, https://github.com/googleapis/google-cloud-python/issues/new?assignees=&labels=&projects=&template=bug_report.md

One benefit of this is that we can set the labels parameter in the url which is empty by default.

For example, we could add the type: bug label and api: access approval label , using url encoding for the colon and space characters.
https://github.com/googleapis/google-cloud-python/issues/new?assignees=&labels=type%3A%20bug%2Capi%3A%20accessapproval&projects=&template=bug_report.md

As long as the priority remains unset, the bug will be considered un-triaged and will still appear in the un-triaged list.

[vchudnov-g]

The links to the new issue templates (#12710) explicitly include the labels I configured in the URL. So for bugs, we can use that URL (which already includes the bug label) and just add the api: XXX label. Note that the proposed bug template will also ask users to fill in the API name, which will be useful ifthey get there not via the README.