[stable12] Show warning if PHP 7.2 is used

[LukasReschke]

Ref #5791

[codecov]

Codecov Report

Merging #5792 into stable12 will decrease coverage by <.01%.
The diff coverage is 0%.

@@              Coverage Diff               @@
##             stable12    #5792      +/-   ##
==============================================
- Coverage       54.09%   54.09%   -0.01%     
+ Complexity      22445    22440       -5     
==============================================
  Files            1380     1380              
  Lines           85830    85820      -10     
  Branches         1329     1329              
==============================================
- Hits            46434    46421      -13     
- Misses          39396    39399       +3

Impacted Files	Coverage Δ	Complexity Δ
index.php	0% <0%> (ø)	0 <0> (ø)	⬇️
apps/encryption/lib/Util.php	77.55% <0%> (-14.29%)	16% <0%> (ø)
apps/encryption/lib/Crypto/Encryption.php	83.43% <0%> (-0.5%)	55% <0%> (-2%)
config/config.sample.php	0% <0%> (ø)	0% <0%> (ø)	⬇️
core/js/js.js	61.94% <0%> (+0.11%)	0% <0%> (ø)	⬇️
lib/private/Mail/Mailer.php	64.17% <0%> (+1.51%)	22% <0%> (-3%)	⬇️
apps/comments/lib/EventHandler.php	87.5% <0%> (+8.33%)	7% <0%> (ø)	⬇️

[MorrisJobke]

I added the same check to console.php and tested it 👍