"Ignore tags older than" ignores tag date, uses commit date instead

### Jenkins and plugins versions report

Environment

```text
Jenkins: 2.568.1
OS: Linux - 6.12.95+deb13-cloud-arm64
Java: 21.0.11 - Debian (OpenJDK 64-Bit Server VM)
---
Office-365-Connector:5.3.0
Parameterized-Remote-Trigger:3.2.2
ace-editor:1.1
active-directory:2.41.2
amazon-ecr:1.161.v1a_1e8df852d6
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aws-java-sdk-elasticloadbalancingv2:1.12.780-480.v4a_0819121a_9e
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flatpickr-api:4.6.13-32.v60a_51029c136
flexible-publish:0.16.1
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git:5.10.1
git-client:6.6.1
git-parameter:462.463.v496a_59f698e5
git-server:137.ve0060b_432302
git-userContent:1.4
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gradle:2.19.1244.v1f9866817fec
groovy:537.v741a_5a_f1b_581
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gson-api:2.14.0-201.v8eefe5515533
hashicorp-vault-plugin:383.v7529eea_ef531
hp-application-automation-tools-plugin:26.2
htmlpublisher:427.1
instance-identity:203.v15e81a_1b_7a_38
ionicons-api:94.vcc3065403257
ivy:604.v6dd214d04ef3
jackson-annotations2-api:2.22-19.v10a_a_582ea_26e
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jackson3-api:3.2.1-92.vd25c2e23c180
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javadoc:354.vee1a_660b_4990
javax-activation-api:1.2.0-8
javax-mail-api:1.6.2-11
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jquery:1.12.4-3
jquery3-api:3.7.1-687.v68d468e40b_30
jsch:0.2.16-95.v3eecb_55fa_b_78
json-api:20260522-217.v0b_18b_8cd4672
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resource-disposer:0.25
run-condition:276.v97298f3a_cd51
s3:520.v4381b_80b_f969
saferestart:102.v4dc1b_9636a_ee
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script-security:1402.1405.vc96e74964250
sidebar-link:2.5.0
snakeyaml-api:2.5-149.v72471e9c6371
snakeyaml-engine-api:3.0.1-5.vd98ea_ff3b_92e
ssh-agent:396.vcc7d84e622ec
ssh-credentials:372.va_250881b_08cd
ssh-slaves:3.1097.v868116049892
sshd:3.384.vc89b_5e138cf9
structs:362.va_b_695ef4fdf9
swarm:1254.vf26a_5e188f26
testng-plugin:981.v1dc64d227855
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throttle-concurrents:625.vc8b_e469e9a_b_c
timestamper:1.30
token-macro:477.vd4f0dc3cb_cf1
trilead-api:2.284.v1974ea_324382
uno-choice:2.8.9
validating-string-parameter:345.v0f4330488a_f6
variant:70.va_d9f17f859e0
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workflow-cps:4350.vcc65d4958821
workflow-durable-task-step:1479.v56e587f413a_7
workflow-job:1581.ve4b_d0db_fcb_b_b_
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workflow-step-api:724.v538c2362b_dfb_
workflow-support:1015.v785e5a_b_b_8b_22
ws-cleanup:0.49
```

### What Operating System are you using (both controller, and any agents involved in the problem)?

See above

### Reproduction steps

1. For a given MultiBranch Pipeline, attach a new annotated tag to a commit that is older than "Ignore tags older than" threshold
2. Push the new tag

### Expected Results

I expect the new tag to be built by the pipeline, because the help text for "Ignore tags older than" clearly says
> The number of days **since the tag was created** after which it is no longer eligible for automatic building.

It does NOT say "since the tagged commit was created".

Annotated tags do have a tag creation date, so it is possible follow what the documentation says, for example:
```
% git show SOME_TAG
tag SOME_TAG
Tagger: Some User
Date: Tue Aug 18 08:40:54 2026 +0000

Comment of annotated tag.
commit (HEAD, tag: SOME_TAG, origin/main, origin/HEAD, main)
Merge: 1234567 7654321
Author: Another User
Date: Mon Aug 10 07:38:13 2026 +0200

Merge branch 'some_branch' into 'main'
```

### Actual Results

The newly created tag is not built, because the tagged commit is older than the threshold. Scan log contains these lines:
```
Checking tag SOME_TAG

‘Jenkinsfile’ found

Met criteria

Changes detected: SOME_TAG (null → )

No automatic build triggered for SOME_TAG
```

### Anything else?

IMHO, the scan log output should also contain the reason for not triggering a build.

### Are you interested in contributing a fix?

Lacking the needed skills, sorry.

Start by locating the workflow-multibranch-plugin implementation and tests for the “Ignore tags older than” setting, then trace how a newly pushed annotated tag is dated during scanning. Compare the tagger date with the tagged commit date and define tests covering an old commit with a new tag; done means the documented date is used and the scan log explains skipped builds.

Written by the indexing model from the issue text.