Closes #6606 Display notice for 3.16 changes

[remyperona]

Description

Display a notice about 3.16 changes on the WPR settings page.

To be able to handle the display of the notice depending on the previous version, We now store the previous version value with the plugin settings.

Fixes #6606

Type of change

• Enhancement (non-breaking change which improves an existing functionality).

Checklists

Feature validation

• I validated all the Acceptance Criteria. If possible, provide sreenshots 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 self-explanatory code about what it does.
• I wrote comments to explain why it does it.
• I named variables and functions explicitely.
• I protected entry points against unexpected inputs.
• I did not introduce unecessary complexity.

Observability

• I handled errors when needed.
•  I wrote user-facing messages that are understandable and provide actionable feedbacks.
• I prepared ways to observe the implemented system (logs, data, etc.).

[codacy-production]

Coverage summary from Codacy

See diff coverage on Codacy

Coverage variation	Diff coverage
✅ +0.08% (target: -0.10%)	✅ 87.50% (target: 50.00%)

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (c89f98a)	37115	14321	38.59%
Head commit (53d1543)	37175 (+60)	14374 (+53)	38.67% (+0.08%)

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 (#6608)	56	49	87.50%

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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[remyperona]

Current status of the PR handles most of the ACs, but:

• The message will be displayed only when updating from the version lower than 3.16. (3.15.x to 3.16 displayed; 3.16 to 3.16.0.1 not displayed; 3.15.x to 3.17 displayed;)
• The message will not be displayed for fresh installations
• Missing IDs and links+IDs FR for the documentation

The first 2 points are waiting brainstorming on how to handle them in the related issue

[remyperona]

Above points are fixed, PR ready for review

[Miraeld]

I've tested and it works fine:

While I upgraded from version 3.15.10 to a version based on this branch, I get the following notice:

Once dismissed, it's not shown again.