Update CI providers and setup

[ararslan]

This replaces Drone and Travis with GitHub Actions and an additional step in the Cirrus matrix. Travis is no longer posting build statuses to this repository and Drone lists every job as "killed" with no explanation. GHA and Cirrus are reliable alternatives.

This commit encompasses the following changes:

• Linux x86-64 with gcc now builds on GHA (was both Travis and Drone)
• Linux ARM64 with gcc now builds on Cirrus (was Drone)
• macOS with clang now builds on GHA (was Travis) but it's important to note that macos-latest uses Apple's ARM-based processors rather than the legacy x86-64 Intel processors. That is, the macOS build on CI is arm64-apple-darwin rather than x86_64-apple-darwin.
• Linux x86-64 with clang now builds on GHA (was Travis)
• FreeBSD 12.3 is removed from CI as that version is EOL
• FreeBSD 13.1 is replaced with 13.3
• FreeBSD 13 and 14 now use release images rather than development snapshots. Notably, development snapshot images seem to be culled for versions no longer in development, which is why the 12.3 and 13.1 builds were unable to locate the specified images.
• tools/ci-build.sh now attempts to determine the core count automatically. There is too much variability across CI providers for --cores=4 to be universally appropriate; as of this writing, GHA uses 4 cores for Linux and 3 for macOS, and Cirrus uses 2.
• Travis and Drone badges in the README are replaced by GHA
• Linux PowerPC with clang is no longer run on CI, so it's removed from the list in the README
• GitHub's Dependabot is added in order to automatically keep the actions used up to date

[ararslan]

The Linux ARM64 build on Cirrus appears to successfully complete and pass all unit tests, but the overall job fails. Cirrus reports that the container was OOM killed, but the memory usage graph shows that memory use is very low for most of the lifetime of the job, even lower than FreeBSD, which completed successfully.

Note that the GitHub Actions jobs will require approval before they'll run here.