Replace deprecated ioutil usage

api/custom/federation.go

api/custom/media_attributes.go

api/r0/upload.go

api/unstable/info.go

cluster/idgen.go

cmd/compile_assets/main.go

cmd/export_synapse_for_import/main.go

cmd/gdpr_import/main.go

cmd/plugin_antispam_ocr/main.go

common/assets/process.go

common/config/access.go

controllers/download_controller/download_controller.go

controllers/download_controller/download_resource_handler.go

controllers/preview_controller/previewers/http.go

...rollers/preview_controller/previewers/oembed_previewer.go

controllers/thumbnail_controller/thumbnail_controller.go

...ollers/thumbnail_controller/thumbnail_resource_handler.go

controllers/upload_controller/upload_controller.go

internal_cache/redis.go

matrix/client_server.go