fix(theme): prevent bold headers in Fiori 3 theme (fixes #4320)

[arunchahal]

Problem
In version 1.136 of OpenUI5, headers in the sap_fiori_3 theme appear excessively bold.
This is caused by the @sapFontHeaderFamily variable resolving to a bold font stack:
"72-Bold", "72-Boldfull", "72", "72full", Arial, Helvetica, sans-serif
This made header text rendering appear too heavy compared to previous versions.

Fix
This PR updates the @sapFontHeaderFamily to use @sapFontFamily, restoring the font behavior seen in versions like 1.138, where the issue no longer occurs.

Impact

• Fixes unintended boldness in headers
• Only affects the sap_fiori_3 theme
• Tested locally with correct font rendering

Additional Notes
This change is isolated to themelib_sap_fiori_3 and aligns with visual expectations across themes.

Fixes #4320

[cla-assistant]

All committers have signed the CLA.

[LidiyaGeorgieva]

Hello @arunchahal ,

The change you are proposing is in the master branch, and you are setting:
@sapFontHeaderFamily to "72", "72full", Arial, Helvetica, sans-serif;,
but this is already the case:
@sapFontHeaderFamily = @sapFontFamily = "72", "72full", Arial, Helvetica, sans-serif (see row 89 from the file base.less)

In the commit message, you said "In version 1.136 of OpenUI5, headers in the sap_fiori_3 theme appear excessively bold.", but if I open Demo Kit in version 1.136.6
https://sapui5.hana.ondemand.com/1.136.6/#/entity/sap.m.IllustratedMessage/sample/sap.m.sample.IllustratedMessageInCardNoEmail
I see the correct font for Fiori 3 (the same as it is in the master)

Please explain your intention in more details and give reproducible examples

Regards,
Lidiya