[chore] update dependencies (#4386)

- codeberg.org/gruf/go-bytesize v1.0.3 -> v1.0.4
- codeberg.org/gruf/go-kv/v2 v2.0.6 -> v2.0.7
- codeberg.org/gruf/go-mutexes v1.5.2 -> v1.5.3
- codeberg.org/gruf/go-structr v0.9.7 -> v0.9.8
- codeberg.org/gruf/go-ffmpreg v0.6.8 -> v0.6.9
- github.com/tomnomnom/linkheader HEAD@2018 -> HEAD@2025

all of the above codeberg.org/gruf updates are in preparation for Go1.25, except for bytesize, and also ffmpreg which is a rebuild with the latest version of ffmpeg (v5.1.7)

Reviewed-on: https://codeberg.org/superseriousbusiness/gotosocial/pulls/4386
Co-authored-by: kim <grufwub@gmail.com>
Co-committed-by: kim <grufwub@gmail.com>

vendor/codeberg.org/gruf/go-mutexes/map_unsafe.go

@@ -1,4 +1,4 @@
-//go:build go1.22 && !go1.25
+//go:build go1.22 && !go1.26
 
 package mutexes
 
@@ -21,7 +21,7 @@ func syncCond_last_ticket(c *sync.Cond) uint32 {
 	// this does not alter the first
 	// 2 fields which are all we need.
 	type notifyList struct {
-		_      atomic.Uint32
+		_      uint32
 		notify uint32
 		// ... other fields
 	}