Add support for the reject_tokens_expiring_in_less_than option.

[ploftness-tesla]

This is a followup pull request, implementing the simplified approach discussed on #365.

[Keats]

Should the docs mention how it interacts with leeway?

[ploftness-tesla]

Yes, I have added some explanation.

[ploftness-tesla]

@Keats Thanks for approving; when do you think this will be merged/released?

[ploftness-tesla]

@Keats I do not have permissions to merge. What are the next steps for this?

[Keats]

I'll merge and release in a bit when i have some free time

[Keats]

Looks like there's another MSRV >_>

[ploftness-tesla]

Looks like there's another MSRV >_>

@Keats Yes, looks like the build error is due to a recent change in a transitive dependency used by some tests.

% cargo tree
jsonwebtoken v9.2.0 (/Users/ploftness/Documents/jsonwebtoken)
...
[dev-dependencies]
...
├── time v0.3.31 (*)
└── wasm-bindgen-test v0.3.40
    ├── console_error_panic_hook v0.1.7
    │   ├── cfg-if v1.0.0
    │   └── wasm-bindgen v0.2.90
    │       ├── cfg-if v1.0.0
    │       └── wasm-bindgen-macro v0.2.90 (proc-macro)
    │           ├── quote v1.0.35 (*)
    │           └── wasm-bindgen-macro-support v0.2.90
...
    │               ├── wasm-bindgen-backend v0.2.90
    │               │   ├── bumpalo v3.14.0
 ...

1. The wasm-bindgen-backend package supports bumpalo ^3.0.0
   https://crates.io/crates/wasm-bindgen-backend/0.2.90/dependencies
2. The bumpalo package recently released some new versions that upgraded their MSRV.
   fitzgen/bumpalo@f8597ce

While this is not strictly connected to this issue, I have gone ahead and bumped the MSRV to get things unblocked.

[ploftness-tesla]

@Keats any updates? Would appreciate getting this released.

[Keats]

Thanks and sorry for the delay

[ploftness-tesla]

@Keats Thanks!