This PR upgrades `tree-sitter-php` to v0.21.1.

Here is the diff between our current version and this version:
https://github.com/tree-sitter/tree-sitter-php/compare/d43130fd1525301e9826f420c5393a4d169819fc...29a49d3a53353444ee2226e2efa140fec69dd3e0

The primary impetus for this change is to get this change that adds the
`license` field to the `Cargo.toml`:
https://github.com/tree-sitter/tree-sitter-php/pull/193

This will silence the warning that is shown when running
`script/generate-licenses`.

Release Notes:

- Upgraded `tree-sitter-php` to v0.21.1.