link::back insert tag is deprecated

[ausi]

back ist deprecated for all link_* insert tags (since contao 4.9.22), see https://github.com/contao/contao/pull/3585

[bezin]

I am sorry for the unrelated references. I made a faulty push to my fork 🙄

[jochi44]

As discussed in https://github.com/contao/contao/issues/6785#issuecomment-1911653307 the deprecation of {{link::back}} should be documented in the user manual.

[Toflar]

@jochi44 would you try to contribute this to the docs yourself? 😊 https://docs.contao.org/manual/en/contributing/