Upgrade framework to Laravel 8

[gggrant]

Resolves #3865.

Notes:

• Requires PHP 7.3
• Also upgrades Backpack to 4.1

[codecov]

Codecov Report

Merging #4535 (d670e8f) into dev (b1968cd) will decrease coverage by 2.54%.
The diff coverage is 11.37%.

Impacted Files	Coverage Δ	Complexity Δ
app/Helpers/Helper.php	91.49% <0.00%> (ø)	0.00 <0.00> (ø)
...Http/Controllers/Admin/ApplicantCrudController.php	3.58% <0.00%> (+3.58%)	2.00 <0.00> (ø)
...Controllers/Admin/ClassificationCrudController.php	12.50% <ø> (+12.50%)	4.00 <0.00> (ø)
...Controllers/Admin/JobApplicationCrudController.php	2.09% <ø> (+2.09%)	3.00 <0.00> (ø)
...ontrollers/Admin/JobPosterStatusCrudController.php	6.25% <0.00%> (+6.25%)	3.00 <0.00> (ø)
.../Admin/JobPosterStatusTransitionCrudController.php	5.00% <0.00%> (+5.00%)	3.00 <0.00> (ø)
.../Controllers/Admin/JobSkillLevelCrudController.php	13.34% <0.00%> (+13.34%)	3.00 <0.00> (ø)
...Http/Controllers/Admin/ResourcesCrudController.php	51.36% <0.00%> (+6.53%)	5.00 <0.00> (ø)
...llers/Admin/TalentStreamCategoryCrudController.php	13.34% <0.00%> (+13.34%)	3.00 <0.00> (ø)
...p/Controllers/Admin/TalentStreamCrudController.php	13.34% <0.00%> (+13.34%)	3.00 <0.00> (ø)
... and 269 more

@@             Coverage Diff              @@
##                dev    #4535      +/-   ##
============================================
- Coverage     49.59%   47.05%   -2.53%     
- Complexity     1796     1804       +8     
============================================
  Files           455      310     -145     
  Lines         10772     6664    -4108     
  Branches        427        0     -427     
============================================
- Hits           5341     3135    -2206     
+ Misses         5116     3529    -1587     
+ Partials        315        0     -315     

Impacted Files	Coverage Δ	Complexity Δ
app/Helpers/Helper.php	91.49% <0.00%> (ø)	0.00 <0.00> (ø)
...Http/Controllers/Admin/ApplicantCrudController.php	3.58% <0.00%> (+3.58%)	2.00 <0.00> (ø)
...Controllers/Admin/ClassificationCrudController.php	12.50% <ø> (+12.50%)	4.00 <0.00> (ø)
...Controllers/Admin/JobApplicationCrudController.php	2.09% <ø> (+2.09%)	3.00 <0.00> (ø)
...ontrollers/Admin/JobPosterStatusCrudController.php	6.25% <0.00%> (+6.25%)	3.00 <0.00> (ø)
.../Admin/JobPosterStatusTransitionCrudController.php	5.00% <0.00%> (+5.00%)	3.00 <0.00> (ø)
.../Controllers/Admin/JobSkillLevelCrudController.php	13.34% <0.00%> (+13.34%)	3.00 <0.00> (ø)
...Http/Controllers/Admin/ResourcesCrudController.php	51.36% <0.00%> (+6.53%)	5.00 <0.00> (ø)
...llers/Admin/TalentStreamCategoryCrudController.php	13.34% <0.00%> (+13.34%)	3.00 <0.00> (ø)
...p/Controllers/Admin/TalentStreamCrudController.php	13.34% <0.00%> (+13.34%)	3.00 <0.00> (ø)
... and 269 more

[Khristophor]

Looks like Backpack added a check that makes us unable to reference custom Model methods as column names:
https://github.com/Laravel-Backpack/CRUD/blob/915edda4cec9abbc7437534cfa01796c15f22e6c/src/app/Library/CrudPanel/Traits/Columns.php#L380-L386

I can try opening a PR against Backpack, or we need to figure out another way to display computed states