Drop PHP7.3 and Composer 1 support

[T2L]

Fixes #251

[codecov]

Codecov Report

Merging #259 (800aa03) into 3.x.x (62ffada) will decrease coverage by 0.13%.
The diff coverage is 91.66%.

@@             Coverage Diff              @@
##              3.x.x     #259      +/-   ##
============================================
- Coverage     94.60%   94.47%   -0.14%     
- Complexity      166      167       +1     
============================================
  Files            26       26              
  Lines           408      398      -10     
============================================
- Hits            386      376      -10     
  Misses           22       22              

Impacted Files	Coverage Δ
src/Config/Install/Step/ProjectName.php	0.00% <ø> (ø)
...Config/Update/Step/AddIdToSyncedFoldersOptions.php	100.00% <ø> (ø)
src/Config/Update/Step/AllowAllHostsMysql.php	100.00% <ø> (ø)
src/Config/Update/Step/Cleanup30400.php	100.00% <ø> (ø)
...fig/Update/Step/RemoveConfigurerComposerScript.php	100.00% <ø> (ø)
...p/ReplaceBaseDirectoryWithDestinationDirectory.php	100.00% <ø> (ø)
src/Config/Update/Step/RevisePhpConfiguration.php	100.00% <ø> (ø)
...c/Config/Update/Step/SetAsAlreadyInstalledStep.php	100.00% <ø> (ø)
src/Config/Update/Step/Xdebug2To3.php	94.44% <ø> (ø)
src/Config/Update/Step/Xenial2Focal.php	100.00% <ø> (ø)
... and 14 more

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