I posted this a while back, but you could try this:
Here is what I did:
clear
ls -l /web/templates/httpd-ssl.conf.templ
#Note: Above just confirms permissions set back to read-only.
cp /web/templates/httpd-ssl.conf.templ /web/templates/httpd-ssl.conf.templ_ORIGINAL
chmod u+w /web/templates/httpd-ssl.conf.templ
sed -i 's/SSLCipherSuite HIGH:!RC4:!LOW:!EXP:!aNULL:!SSLv2:!MD5/SSLCipherSuite ECDHE-RSA-AES256-SHA384:AES256-SHA256:!ADH:!EXP:!RSA:+HIGH:+MEDIUM:!MD5:!LOW:!NULL:!SSLv2:!eNULL:!aNULL:!RC4:!SHA1/g' /web/templates/httpd-ssl.conf.templ
sed -i 's/SSLProtocol -ALL {ifcmp = $httpd:ssl3_enabled 1}+{else}-{endif}SSLv3 +TLSv1 +TLSv1.1 +TLSv1.2/SSLProtocol -ALL {ifcmp = $httpd:ssl3_enabled 1}+{else}-{endif}TLSv1.2 +TLSv1.3/g' /web/templates/httpd-ssl.conf.templ
chmod u-w /web/templates/httpd-ssl.conf.templ
/bin/template_xlate : /web/templates/httpd-ssl.conf.templ /web/conf/extra/httpd-ssl.conf < /config/active
tellpm process:httpd2
tellpm process:httpd2 t
ls -l /web/templates/httpd-ssl.conf.templ
#Note: Above just confirms permissions set back to read-only.
I then ran an sslscan against the IP which resulted in only TLSv1.3 being seen.
Testing SSL server aa.bb.cc.dd on port 443 using SNI name aa.bb.cc.dd
SSL/TLS Protocols:
SSLv2 disabled
SSLv3 disabled
TLSv1.0 disabled
TLSv1.1 disabled
TLSv1.2 disabled
TLSv1.3 enabled
TLS Fallback SCSV:
Server supports TLS Fallback SCSV
TLS renegotiation:
Session renegotiation not supported
TLS Compression:
Compression disabled
Heartbleed:
TLSv1.3 not vulnerable to heartbleed
Supported Server Cipher(s):
Preferred TLSv1.3 128 bits TLS_AES_128_GCM_SHA256 Curve 25519 DHE 253
Accepted TLSv1.3 256 bits TLS_AES_256_GCM_SHA384 Curve 25519 DHE 253
Accepted TLSv1.3 256 bits TLS_CHACHA20_POLY1305_SHA256 Curve 25519 DHE 253
Server Key Exchange Group(s):
TLSv1.3 128 bits secp256r1 (NIST P-256)
TLSv1.3 192 bits secp384r1 (NIST P-384)
TLSv1.3 260 bits secp521r1 (NIST P-521)
TLSv1.3 128 bits x25519
TLSv1.3 224 bits x448
What I'm not sure about is if this procedure would need to run again after updating the jumbo.