Do not allow setting never-settable attributes (+avoid new warnings caused by new Rust version)

[Jakuje]

The never settable attribute was not handled and did allow changing attributes that were never settable (even the OBJECT_VALIDATION, even though it is ephemeral).

Additionally the Rust was updated showing new warnings:

```
warning: hiding a lifetime that's elided elsewhere is confusing
  --> ossl/src/asymcipher.rs:30:25
   |
30 |     oaep_params: Option<&RsaOaepParams>,
   |                         ^^^^^^^^^^^^^^ the lifetime is elided here
31 | ) -> Result<OsslParam, Error> {
   |             --------- the same lifetime is hidden here
   |
   = help: the same lifetime is referred to in inconsistent ways, making the signature confusing
   = note: `#[warn(mismatched_lifetime_syntaxes)]` on by default
help: use `'_` for type paths
   |
31 | ) -> Result<OsslParam<'_>, Error> {
   |                      ++++
```
and similar.

It also turns out the `from_attr_slice` is used only from the nssdb:

```
warning: associated function `from_attr_slice` is never used
   --> src/attribute.rs:465:12
    |
295 | impl Attribute {
    | -------------- associated function in this implementation
...
465 |     pub fn from_attr_slice(
    |            ^^^^^^^^^^^^^^^
    |
    = note: `#[warn(dead_code)]` on by default
```

#### Description

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Individual commit message should highlight important changes that people may
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#### Checklist

~- [ ] Test suite updated with functionality tests~
~- [ ] Test suite updated with negative tests~
~- [ ] Rustdoc string were added or updated~
~- [ ] CHANGELOG and/or other documentation added or updated~
~- [ ] This is not a code change~

#### Reviewer's checklist:

- [ ] Any issues marked for closing are fully addressed
- [ ] There is a test suite reasonably covering new functionality or modifications
- [ ] This feature/change has adequate documentation added
- [ ] A changelog entry is added if the change is significant
- [ ] Code conform to coding style that today cannot yet be enforced via the check style test
- [ ] Commits have short titles and sensible text
- [ ] Doc string are properly updated

[simo5]

LGTM