[chore] update go-structr and go-mangler to no longer rely on modern-go/reflect2

[NyaaaWhatsUpDoc]

Description

Updates go-structr and go-mangler to no longer rely on modern-go/reflect2, which come release of Go 1.23 with changes to how linkname works (now requires both sides of a linkage to confirm it), means we will no longer be reliant on weird build tags to revert to old linkname behaviour.

Checklist

• I/we have read the GoToSocial contribution guidelines.
• I/we have discussed the proposed changes already, either in an issue on the repository, or in the Matrix chat.
• I/we have not leveraged AI to create the proposed changes.
• I/we have performed a self-review of added code.
• I/we have written code that is legible and maintainable by others.
• I/we have commented the added code, particularly in hard-to-understand areas.
• I/we have made any necessary changes to documentation.
• I/we have added tests that cover new code.
• I/we have run tests and they pass locally with the changes.
• I/we have run go fmt ./... and golangci-lint run.

[tsmethurst]

If you're happy then squerge :)

[NyaaaWhatsUpDoc]

the main diffs:
go-mangler changes: https://codeberg.org/gruf/go-mangler/compare/v1.3.0...v1.4.0
go-structr changes: https://codeberg.org/gruf/go-structr/compare/v0.8.5...v0.8.6