[R-package] CRAN requires a new release by July 30

[jameslamb]

Description

At https://cran.r-project.org/web/checks/check_results_lightgbm.html, checks show this warning:

Version: 4.6.0
Check: whether package can be installed
Result: WARN
Found the following significant warnings:
Warning: src/Makevars.win: Unknown C++ standard '@CXX_STD@' was ignored
See 'd:/Rcompile/CRANpkg/local/4.7/lightgbm.Rcheck/00install.out' for details.

• used C++ compiler: 'g++.exe (GCC) 14.3.0'
  Flavor: r-devel-windows-x86_64

I received an email tonight saying that we must do a release to resolve this (in my opinion, minor) warning by July 30 or risk the project being archived 🙃

Approach

Thankfully, we already fixed this in source control (#7043) and are close to ready with the next release anyway: #7129