pin haproxy version to 2.8.5-1ubuntu3.3

[Thanhphan1147]

With 2.8.5-1ubuntu3.3 available, running apt install -y haproxy=2.8.5-1ubuntu3 will result in the following error:

charms.operator_libs_linux.v0.apt.PackageError: Could not install package(s) ['haproxy=2.8.5-1ubuntu3']: E: Packages were downgraded and -y was used without --allow-downgrades.

Also added a call to apt-mark hold haproxy to pin the haproxy version and prevent automatic updates.

Checklist

• The charm style guide was applied
• The contributing guide was applied
• The changes are compliant with ISD054 - Managing Charm Complexity
• The documentation is generated using src-docs
• The documentation for charmhub is updated
• The PR is tagged with appropriate label (urgent, trivial, complex)

[github-actions]

Test results for commit d2ba585

Test coverage for d2ba585

Name                         Stmts   Miss Branch BrPart  Cover   Missing
------------------------------------------------------------------------
src/charm.py                   208     85     56      8    52%   87, 163, 171-184, 189, 194, 204, 223-245, 249-271, 282-283, 311-316, 323, 336-342, 348, 351, 358, 370-384, 389-392, 397, 405-420, 429-435
src/haproxy.py                  79     22      2      1    72%   86, 103-108, 123-130, 143-151, 159-165, 174-183, 193-194, 222, 246-248
src/http_interface.py           73     25      4      0    62%   74, 83, 92, 106-108, 126, 138, 150, 162, 170-175, 187, 194, 202, 217-227
src/state/config.py             41     12      6      3    68%   53, 60-62, 67-68, 71, 93-95, 107-108
src/state/exception.py           1      0      0      0   100%
src/state/ha.py                 30      1      2      1    94%   50
src/state/haproxy_route.py      98     15     24      4    80%   102, 118, 141-145, 159-167, 215, 251, 259
src/state/ingress.py            35      0      4      0   100%
src/state/tls.py                38      5      8      3    83%   69, 72-73, 109-110
src/state/validation.py         38     16      6      1    52%   65-66, 70-83
src/tls_relation.py             48      8     18      7    77%   60, 62, 64, 74-75, 77->76, 100-106, 118->120
------------------------------------------------------------------------
TOTAL                          689    189    130     28    68%

Static code analysis report

Run started:2025-06-26 10:22:48.656337

Test results:
  No issues identified.

Code scanned:
  Total lines of code: 3037
  Total lines skipped (#nosec): 3
  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):