Use PETSc version 3.25.5

[yguclu]

Following the release of PETSc version 3.25.5 on August 30, 2026:

• Update PETSc version in docs/installation.md from 3.25.0 to 3.25.5
• Download PETSc 3.25.5 in GitHub workflows

Additional changes:

• Update GitHub actions checkout and setup-python to version 6 in all workflows

[codacy-production]

Up to standards ✅

🟢 Issues 0 issues

Results:
0 new issues

View in Codacy

🟢 Coverage ∅ diff coverage

Metric	Results
Coverage variation	Report missing for 746d0c91
Diff coverage	✅ ∅ diff coverage

View coverage diff in Codacy

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (746d0c9)	Report Missing	Report Missing	Report Missing
Head commit (a4f8631)	49470	32898	66.50%

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 (#595)	0	0	∅ (not applicable)

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%

¹ Codacy didn't receive coverage data for the commit, or there was an error processing the received data. Check your integration for errors and validate that your coverage setup is correct.

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TIP This summary will be updated as you push new changes.}

[kvrigor]

All good!