[Tests-Only] Adjust code and settings so phpstan 0.12.26 passes

[phil-davis]

Description

In phpstan.neon for 0.12.26 :

• bootstrap becomes bootstrapFiles
• autoload_directories is no longer needed
• ignore various code like apps/files_external/lib/Lib/Backend/DAV.php that report problems - see issue LegacyDependencyCheckPolyfill trait has abstract getStorageClass #37498

This gets CI passing again, and the issues can be looked at separately.

Related Issue

• Fixes phpstan failing #37495

How Has This Been Tested?

CI

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Database schema changes (next release will require increase of minor version instead of patch)
• Breaking change (fix or feature that would cause existing functionality to change)
• Technical debt
• Tests only (no source changes)

Checklist:

• Code changes
• Unit tests added
• Acceptance tests added
• Documentation ticket raised:
• Changelog item, see TEMPLATE

[codecov]

Codecov Report

Merging #37497 into master will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master   #37497   +/-   ##
=========================================
  Coverage     64.68%   64.68%           
  Complexity    19336    19336           
=========================================
  Files          1277     1277           
  Lines         75535    75535           
  Branches       1331     1331           
=========================================
  Hits          48860    48860           
  Misses        26283    26283           
  Partials        392      392           

Flag	Coverage Δ	Complexity Δ
#javascript	54.14% <ø> (ø)	0.00 <ø> (ø)
#phpunit	65.84% <ø> (ø)	19336.00 <ø> (ø)

Impacted Files	Coverage Δ	Complexity Δ
lib/private/legacy/image.php	47.15% <ø> (ø)	170.00 <0.00> (ø)
lib/private/legacy/util.php	74.53% <ø> (ø)	219.00 <0.00> (ø)

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[phil-davis]

Also see #37500 for the next changes to make phpstan 0.12.27 pass.