Upgrade google.golang.org/protobuf to v1.33.0

README.md: remove old warning about ext4 encryption on removable media

This warning is only applicable for systems with kernel versions older
than v4.1, which is now too old to be worth the mention here.

README.md: update documentation about alternatives

ci.yml: use actions/checkout@v4

This addresses the following warning from GitHub Actions:

    Node.js 16 actions are deprecated. Please update the following
    actions to use Node.js 20: actions/checkout@v3. For more information
    see:
    https://github.blog/changelog/2023-09-22-github-actions-transitioning-from-node-16-to-node-20/.

A near-identical warning forced everyone to upgrade to v3 last year, so
this is some pointless churn, but let's just get it over with again...

build(deps): bump golang.org/x/crypto from 0.13.0 to 0.17.0

Bumps [golang.org/x/crypto](https://github.com/golang/crypto) from 0.13.0 to 0.17.0.
- [Commits](https://github.com/golang/crypto/compare/v0.13.0...v0.17.0)

---
updated-dependencies:
- dependency-name: golang.org/x/crypto
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>