chore: migrate from self-hosted edge runner to GitHub-hosted runner

[cbartz]

Summary

This PR migrates unit and integration test workflows from the self-hosted amd64-noble-medium-edge-ps7 ("edge") runner to GitHub-hosted ubuntu-24.04 runners, which are free and unlimited for public repositories.

The self-hosted-runner and self-hosted-runner-label inputs are removed from the reusable workflow calls; the operator-workflows default then falls back to GitHub-hosted runners.

[github-actions]

Test results for commit 1c6ece2

Test coverage for 1c6ece2

Name            Stmts   Miss Branch BrPart  Cover   Missing
-----------------------------------------------------------
src/charm.py       47      0      6      0   100%
src/errors.py       1      0      0      0   100%
src/state.py      139      0     32      0   100%
-----------------------------------------------------------
TOTAL             187      0     38      0   100%

Static code analysis report

Run started:2026-04-30 12:26:27.845666+00:00

Test results:
  No issues identified.

Code scanned:
  Total lines of code: 986
  Total lines skipped (#nosec): 0
  Total potential issues skipped due to specifically being disabled (e.g., #nosec BXXX): 0

Run metrics:
  Total issues (by severity):
  	Undefined: 0
  	Low: 0
  	Medium: 0
  	High: 0
  Total issues (by confidence):
  	Undefined: 0
  	Low: 0
  	Medium: 0
  	High: 0
Files skipped (0):

[github-actions]

Test results for commit 1c6ece2

Test coverage for 1c6ece2

Name                   Stmts   Miss Branch BrPart  Cover   Missing
------------------------------------------------------------------
src/certificates.py       21      2      6      2    85%   65-68
src/charm.py             106     12      8      0    89%   64-65, 120, 149, 153-156, 211-213, 223-225
src/errors.py             16      0      0      0   100%
src/nginx_manager.py     199     36     16      4    81%   207, 213-219, 224-226, 229, 277-279, 295-297, 337-342, 417-419, 438-442, 461->463, 543-548
src/state.py             163     13     34      1    92%   154-159, 429-439
src/utilities.py          13      0      0      0   100%
------------------------------------------------------------------
TOTAL                    518     63     64      7    88%

Static code analysis report

Run started:2026-04-30 12:26:33.161182+00:00

Test results:
  No issues identified.

Code scanned:
  Total lines of code: 1208
  Total lines skipped (#nosec): 0
  Total potential issues skipped due to specifically being disabled (e.g., #nosec BXXX): 2

Run metrics:
  Total issues (by severity):
  	Undefined: 0
  	Low: 0
  	Medium: 0
  	High: 0
  Total issues (by confidence):
  	Undefined: 0
  	Low: 0
  	Medium: 0
  	High: 0
Files skipped (0):