Fix Multisite Cache Deletion Issue with lazyload background images feature

[wordpressfan]

Description

Fixes #6340

Here we unify the caching directory path for lazyload background images feature.

Type of change

• Bug fix (non-breaking change which fixes an issue).

Detailed scenario

Everything is mentioned in the issue itself, u can install multisite and add some background images in some pages (different sub-sites) then in one site try clearing the cache, this will remove the background images for all sub-sites.

Technical description

Here we unify the way that we instantiate "lazyload_css_cache" in the container service provider.

Documentation

N/A

New dependencies

N/A

Risks

I think we will need to clear cache with update to that version because the files urls are changed, but need to confirm that with QA and product.

Mandatory Checklist

Code validation

• I validated all the Acceptance Criteria. If possible, provide screenshots or videos.
• I triggered all changed lines of code at least once without new errors/warnings/notices.
• I implemented built-in tests to cover the new/changed code.

Code style

• I wrote a self-explanatory code about what it does.
• I protected entry points against unexpected inputs.
• I did not introduce unnecessary complexity.
• Output messages (errors, notices, logs) are explicit enough for users to understand the issue and are actionnable.

Unticked items justification

N/A

Additional Checks

• In the case of complex code, I wrote comments to explain it.
• When possible, I prepared ways to observe the implemented system (logs, data, etc.).
• I added error handling logic when using functions that could throw errors (HTTP/API request, filesystem, etc.)

[codacy-production]

Coverage summary from Codacy

See diff coverage on Codacy

Coverage variation	Diff coverage
Report missing for e726c3d1	✅ 54.55% (target: 50.00%)

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (e726c3d)	Report Missing	Report Missing	Report Missing
Head commit (fefa3b0)	38768	17148	44.23%

Coverage variation is the difference between the coverage for the head and common ancestor commits of the pull request branch: <coverage of head commit> - <coverage of common ancestor commit>

Diff coverage details

	Coverable lines	Covered lines	Diff coverage
Pull request (#7226)	11	6	54.55%

Diff coverage is the percentage of lines that are covered by tests out of the coverable lines that the pull request added or modified: <covered lines added or modified>/<coverable lines added or modified> * 100%

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Footnotes

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