update SPDX License info

* link to latest license info
* update link for further info to main spdx page
* Update src/doc/src/reference/manifest.md
* point directly at expression syntax

Co-authored-by: Weihang Lo <[email protected]>

update license list

src/doc/src/reference/manifest.md

@@ -258,9 +258,9 @@ The `license` field contains the name of the software license that the package
 is released under. The `license-file` field contains the path to a file
 containing the text of the license (relative to this `Cargo.toml`).
 
-[crates.io] interprets the `license` field as an [SPDX 2.1 license
-expression][spdx-2.1-license-expressions]. The name must be a known license
-from the [SPDX license list 3.11][spdx-license-list-3.11]. Parentheses are not
+[crates.io] interprets the `license` field as an [SPDX 2.3 license
+expression][spdx-2.3-license-expressions]. The name must be a known license
+from the [SPDX license list 3.20][spdx-license-list-3.20]. Parentheses are not
 currently supported. See the [SPDX site] for more information.
 
 SPDX license expressions support AND and OR operators to combine multiple
@@ -628,9 +628,9 @@ more detail.
 [docs.rs]: https://docs.rs/
 [publishing]: publishing.md
 [Rust Edition]: ../../edition-guide/index.html
-[spdx-2.1-license-expressions]: https://spdx.org/spdx-specification-21-web-version#h.jxpfx0ykyb60
-[spdx-license-list-3.11]: https://github.com/spdx/license-list-data/tree/v3.11
-[SPDX site]: https://spdx.org/license-list
+[spdx-2.3-license-expressions]: https://spdx.github.io/spdx-spec/v2.3/SPDX-license-expressions/
+[spdx-license-list-3.20]: https://github.com/spdx/license-list-data/tree/v3.20
+[SPDX site]: https://spdx.org
 [TOML]: https://toml.io/
 
 <script>