feat: require PHP7.4

[phanan]

PHP 7.4 has been released a year and a half ago. Time to upgrade.

[codecov-commenter]

Codecov Report

Merging #1326 (cb12f16) into master (d30c598) will increase coverage by 0.14%.
The diff coverage is 75.53%.

@@             Coverage Diff              @@
##             master    #1326      +/-   ##
============================================
+ Coverage     70.49%   70.63%   +0.14%     
+ Complexity      481      480       -1     
============================================
  Files            89       88       -1     
  Lines          1403     1369      -34     
============================================
- Hits            989      967      -22     
+ Misses          414      402      -12     

Impacted Files	Coverage Δ
app/Factories/StreamerFactory.php	100.00% <ø> (ø)
app/Helpers.php	81.25% <0.00%> (+4.77%)	⬆️
app/Http/Controllers/API/AlbumCoverController.php	100.00% <ø> (ø)
.../Http/Controllers/API/AlbumThumbnailController.php	100.00% <ø> (ø)
app/Http/Controllers/API/ArtistImageController.php	100.00% <ø> (ø)
app/Http/Controllers/API/AuthController.php	0.00% <ø> (ø)
app/Http/Controllers/API/DataController.php	0.00% <ø> (ø)
...pp/Http/Controllers/API/Interaction/Controller.php	100.00% <ø> (ø)
...llers/API/Interaction/RecentlyPlayedController.php	0.00% <ø> (ø)
app/Http/Controllers/API/LastfmController.php	100.00% <ø> (ø)
... and 62 more

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[cypress]

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Test summary

104 • 4 • 0 • 0 • 7

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Run details

Project	koel
Status	Failed
Commit	cb12f16
Started	Jun 4, 2021 10:38 PM
Ended	Jun 4, 2021 10:41 PM
Duration	03:07 💡
OS	Linux Ubuntu - 20.04
Browser	Electron 89

View run in Cypress Dashboard ➡️

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Failures

	albums.spec.ts		1
1	Albums > invokes artist screen
	artists.spec.ts		2
1	Artists > loads the list of artists
2	Artists > plays all songs by an artist
	favorites.spec.ts		1
1	Favorites > deletes a favorite with Backspace key

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Flakiness

	albums.spec.ts		2
1	Albums > loads the list of albums
2	Albums > changes display mode
	artists.spec.ts		1
1	Artists > changes display mode
	queuing.spec.ts		2
1	Queuing > navigates through the queue
2	Queuing > rotates if reaches end of queue in repeat-all mode
This comment includes only the first 5 flaky tests. See all 7 flaky tests in the Cypress Dashboard.

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