Update signing config and trusted root to latest versions (#252)

We're actively turning down alpha2.

Signed-off-by: Hayden <[email protected]>
Co-authored-by: Hayden <[email protected]>

Add checkpoint signature mismatch test for Rekor v2 (#250)

Signed-off-by: Aaron Lew <[email protected]>

Add timestamp-related Rekor v2 tests (#235)

Signed-off-by: Aaron Lew <[email protected]>

Prepare for 0.0.18 release (#205)

Changelog is
* Use current sigstore-protobuf-specs
* Make sure the test suite is installed in a separate environment in
  both Makefile and the action

Signed-off-by: Jussi Kukkonen <[email protected]>

README: prep for v0.0.17 release (#200)

handle different certificate fields correctly (#176)

Get ready for 0.0.14 release (#174)

README: prep 0.0.13 (#171)

README: prep 0.0.12 (#167)