Add help messages for tag time boundaries

Signed-off-by: dodo920306 <dodo920306@gmail.com>

Author: dodo920306

src/main/resources/jenkins/plugins/git/traits/TagDiscoveryTrait/help-atLeastDays.html

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+<div>
+    Minimum age of tags (in days) to <strong>include</strong> when discovering tags.
+    Tags newer than this value are ignored.
+    Tag age is calculated from the tag creation / commit timestamp to the time the
+    scan runs.
+    Leave this field blank to <strong>not</strong> filter out tags based on a minimum age
+    (no lower age bound).
+    Examples:
+    <ul>
+        <li><code>7</code> – ignore tags created in the last 7 days (only tags at least 7 days old are used).</li>
+        <li><code>0</code> – do not ignore tags for being too new (equivalent to leaving it blank).</li>
+    </ul>
+    When used together with <em>Ignore tags older than</em> (<code>atMostDays</code>),
+    this value should be less than or equal to the <em>older than</em> value to define a valid age range.
+</div>

src/main/resources/jenkins/plugins/git/traits/TagDiscoveryTrait/help-atMostDays.html

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+<div>
+    Maximum age of tags (in days) to <strong>include</strong> when discovering tags.
+    Tags older than this value are ignored.
+    Tag age is calculated from the tag creation / commit timestamp to the time the
+    scan runs.
+    Leave this field blank to <strong>not</strong> filter out tags based on a maximum age
+    (no upper age bound).
+    Examples:
+    <ul>
+        <li><code>7</code> – ignore tags older than 7 days (only tags from the last 7 days are used).</li>
+        <li><code>30</code> – ignore tags older than 30 days (only tags from the last 30 days are used).</li>
+    </ul>
+    When used together with <em>Ignore tags newer than</em> (<code>atLeastDays</code>),
+    this value should be greater than or equal to the <em>newer than</em> value so that the
+    age range is consistent.
+</div>