The various `Parse` functions in `jws.go` that construct
`jose.JSONWebSignature` take a slice of acceptable signature algorithms.
We now document its purpose and usage. While I was in there, I also
added a reference to RFC 7515 for detached paylods.

Fixes #162

Previously done in the /v3 branch, this is for `main` (v4)

Define a custom error, ErrUnexpectedSignatureAlgorithm, returned when a
JWS header contains an unsupported signature algorithm.

replace x/crypto/pbkdf2 with crypto/pbkdf2 if go 1.24 is used
avoid one dependency and helps with fips mode

We only use it in a few small places.

Without cmp.Diff, we might not get as nice test failures, but it's not
too hard to use an external diff tool if something were to break.

See also https://go-review.googlesource.com/c/oauth2/+/655455

name                  old time/op    new time/op    delta
ParseSignedCompat-16 1.95µs ± 1% 1.91µs ± 1% -1.83% (p=0.000 n=9+10)

name                  old alloc/op   new alloc/op   delta
ParseSignedCompat-16 3.26kB ± 0% 3.21kB ± 0% -1.47% (p=0.000 n=10+10)

name                  old allocs/op  new allocs/op  delta
ParseSignedCompat-16 45.0 ± 0% 44.0 ± 0% -2.22% (p=0.000 n=10+10)

Added a missing quote to the unknown curve error message. 

Co-authored-by: Sudhanva Hebbale <[email protected]>

Previously, this was checking the `enc` header when `alg` was non-empty.

Follow-up to 9b5800e, "Require specifying algorithms for ParseEncrypted"
(#69)

- Uses a build tag to decide between stdlib pbkdf2 implementation or
`x/` legacy implementation based on Go version
- Restores benchmarks to legacy syntax
- Sets go.mod minimum version to 1.23.0
- Since Go 1.21, the `.0` should be added to indicate the minimum
version of the toolchain AFAIK, notably it excludes prereleases
https://tip.golang.org/doc/toolchain#version So I went with it instead
of going to the previous `1.23`
- Restores 1.23 CI workflow

Co-authored-by: kruskall <[email protected]>