Change template engine or increase Java baseline

[shakuzen]

See #224 (comment) for the original context.
We use handlebars for our template engine. It seems supported versions of handlebars going forward will be Java 17 baseline. That means we either need to bump our baseline or change to a different template engine to be on a supported version. It would be ideal to keep our Java baseline for this project in line with the rest of the Micrometer portfolio. In some sense, this project is a little different since it can be run as a build task and therefore does not need to use the same Java version your code is compiled with or targeted to. That said, a Java baseline bump would mean a new major version and it would probably complicate some users' builds that are baselined lower than Java 17.