Revert der-parser update and lock to working version

[tjkirch]

Description of changes:

After the merges of #1654 and #1683, we would get errors on startup from certdog saying there were invalid certificates and boot would fail. I must have done insufficient re-testing after rebasing #1683.

I bisected and found that der-parser is specifically the crate update that causes the failure. As mentioned in the comment below:

# x509-parser depends on der-parser ^5.0.  5.1.1 contains breaking changes.
# The 5.1.1 release isn't in the master branch; those changes are instead in a
# 6.0.0 release, more clearly implying breaking changes.  Lock to 5.1.0.

@arnaldo2792 offered to dive deeper on exactly which change in 5.1.1 is breaking, and communicate upstream or fix our code as necessary. For now, locking this version fixes our issue.

Testing done:

This resolves the certificate error on startup, and the node is otherwise healthy as well.

Terms of contribution:

By submitting this pull request, I agree that this contribution is dual-licensed under the terms of both the Apache License, version 2.0, and the MIT license.

[arnaldo2792]

👢