Use a variable for bold font weight

[janfaracik]

Standardises bold fonts on Jenkins using a CSS variable, overall the UI should look the same - some elements are a little lighter/less heavy on the eye. Overall not very noticeable but a quality of life update - using a variable keeps it consistent and easy to change in the future.

Testing done

• Works as expected, font weights are slightly less bold across Jenkins

Proposed changelog entries

• N/A

Proposed upgrade guidelines

N/A

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1. The Jira issue, if it exists, is well-described.
2. The changelog entries and upgrade guidelines are appropriate for the audience affected by the change (users or developers, depending on the change) and are in the imperative mood (see examples). Fill in the Proposed upgrade guidelines section only if there are breaking changes or changes that may require extra steps from users during upgrade.
3. There is automated testing or an explanation as to why this change has no tests.
4. New public classes, fields, and methods are annotated with @Restricted or have @since TODO Javadocs, as appropriate.
5. New deprecations are annotated with @Deprecated(since = "TODO") or @Deprecated(forRemoval = true, since = "TODO"), if applicable.
6. New or substantially changed JavaScript is not defined inline and does not call eval to ease future introduction of Content Security Policy (CSP) directives (see documentation).
7. For dependency updates, there are links to external changelogs and, if possible, full differentials.
8. For new APIs and extension points, there is a link to at least one consumer.

Desired reviewers

@jenkinsci/sig-ux

Before the changes are marked as ready-for-merge:

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2. Conversations in the pull request are over, or it is explicit that a reviewer is not blocking the change.
3. Changelog entries in the pull request title and/or Proposed changelog entries are accurate, human-readable, and in the imperative mood.
4. Proper changelog labels are set so that the changelog can be generated automatically.
5. If the change needs additional upgrade steps from users, the upgrade-guide-needed label is set and there is a Proposed upgrade guidelines section in the pull request title (see example).
6. If it would make sense to backport the change to LTS, a Jira issue must exist, be a Bug or Improvement, and be labeled as lts-candidate to be considered (see query).

[janfaracik]

Annoyingly links and buttons with the class jenkins-button have different weights, this fixes that. Noticeable on the REST API/Jenkins version buttons.

[janfaracik]

Not really related - but 1.6rem is 25.6px which would look odd as its not a whole number.

[janfaracik]

Adjusted for consistency.

[timja]

LGTM

[timja]

@janfaracik rfe is used as 'enhancement' / 'feature' and goes on the changelog, use skip-changelog for things that aren't user visible.

[krisstern]

/label ready-for-merge

This PR is now ready for merge, after ~24 hours, we will merge it if there's no negative feedback.