Don't use C++20 deprecated implict this captures

These implicit captures are deprecated in C++20.
This was causing compiler warnings like this when compiled with clang:

plasma-dialer/src/call-manager.cpp:60:73: warning: implicit capture of ‘this’ via ‘[=]’ is deprecated in C++20 [-Wdeprecated]
   60 |     connect(d->callChannel.data(), &Tp::CallChannel::invalidated, this, [=]() {